Add table of contents to contributing.md and re-arrange sligthly. (JuliaLang#57575)

- **Add TOC to contributing.md**
- **Re-arrange TOC**
- **Move sections according to TOC movement**
- **Remove H4 headings from TOC**

Author: LilithHafner

CONTRIBUTING.md

@@ -4,37 +4,40 @@ Hi! If you are new to the Julia community: welcome, and thanks for trying Julia.
 
 If you are already familiar with Julia itself, this blog post by Katharine Hyatt on [Making your first Julia pull request](https://kshyatt.github.io/post/firstjuliapr/) is a great way to get started.
 
-## Learning Julia
-
-[The learning page](https://julialang.org/learning) has a great list of resources for new and experienced users alike.
-
-## Before filing an issue
-
-- Reporting a potential bug? Please read the "[How to file a bug report](https://github.com/JuliaLang/julia/blob/master/CONTRIBUTING.md#how-to-file-a-bug-report)" section to make sure that all necessary information is included.
 
-- Contributing code? Be sure to review the [contributor checklist](https://github.com/JuliaLang/julia/blob/master/CONTRIBUTING.md#contributor-checklist) for helpful tips on the tools we use to build Julia.
+# Table of Contents
 
-- Library feature requests are generally not accepted on this issue tracker. New libraries should be developed as [packages](https://julialang.github.io/Pkg.jl/v1/creating-packages/). Discuss ideas for libraries at the [Julia Discourse forum](https://discourse.julialang.org). Doing so will often lead to pointers to existing projects and bring together collaborators with common interests.
+1. [Learning Julia](#learning-julia)
+2. [Filing an issue](#filing-an-issue)
+    - [Before filing an issue](#before-filing-an-issue)
+    - [How to file a bug report](#how-to-file-a-bug-report)
+3. [Submitting contributions](#submitting-contributions)
+    - [Contributor Checklist](#contributor-checklist)
+    - [Writing tests](#writing-tests)
+    - [Improving documentation](#improving-documentation)
+    - [Contributing to core functionality or base libraries](#contributing-to-core-functionality-or-base-libraries)
+    - [Contributing to the standard library](#contributing-to-the-standard-library)
+    - [Contributing to patch releases](#contributing-to-patch-releases)
+    - [Code Formatting Guidelines](#code-formatting-guidelines)
+    - [Git Recommendations For Pull Requests](#git-recommendations-for-pull-requests)
+4. [Resources](#resources)
 
-## Contributor Checklist
-
-* Create a [GitHub account](https://github.com/signup/free).
 
-* [Fork Julia](https://github.com/JuliaLang/julia/fork).
+## Learning Julia
 
-* Build the software and libraries (the first time takes a while, but it's fast after that). Detailed build instructions are in the [README](https://github.com/JuliaLang/julia/tree/master/README.md). Julia depends on several external packages; most are automatically downloaded and installed, but are less frequently updated than Julia itself.
+[The learning page](https://julialang.org/learning) has a great list of resources for new and experienced users alike.
 
-* Keep Julia current. Julia is a fast-moving target, and many details of the language are still settling out. Keep the repository up-to-date and rebase work-in-progress frequently to make merges simpler.
+## Filing an issue
 
-* Learn to use [git](https://git-scm.com), the version control system used by GitHub and the Julia project. Try a tutorial such as the one [provided by GitHub](https://try.GitHub.io/levels/1/challenges/1).
+### Before filing an issue
 
-* Review discussions on the [Julia Discourse forum](https://discourse.julialang.org).
+- Reporting a potential bug? Please read the "[How to file a bug report](https://github.com/JuliaLang/julia/blob/master/CONTRIBUTING.md#how-to-file-a-bug-report)" section to make sure that all necessary information is included.
 
-* For more detailed tips, read the [submission guide](https://github.com/JuliaLang/julia/blob/master/CONTRIBUTING.md#submitting-contributions) below.
+- Contributing code? Be sure to review the [contributor checklist](https://github.com/JuliaLang/julia/blob/master/CONTRIBUTING.md#contributor-checklist) for helpful tips on the tools we use to build Julia.
 
-* Relax and have fun!
+- Library feature requests are generally not accepted on this issue tracker. New libraries should be developed as [packages](https://julialang.github.io/Pkg.jl/v1/creating-packages/). Discuss ideas for libraries at the [Julia Discourse forum](https://discourse.julialang.org). Doing so will often lead to pointers to existing projects and bring together collaborators with common interests.
 
-## How to file a bug report
+### How to file a bug report
 
 A useful bug report filed as a GitHub issue provides information about how to reproduce the error.
 
@@ -56,6 +59,24 @@ A useful bug report filed as a GitHub issue provides information about how to re
 
 ## Submitting contributions
 
+### Contributor Checklist
+
+* Create a [GitHub account](https://github.com/signup/free).
+
+* [Fork Julia](https://github.com/JuliaLang/julia/fork).
+
+* Build the software and libraries (the first time takes a while, but it's fast after that). Detailed build instructions are in the [README](https://github.com/JuliaLang/julia/tree/master/README.md). Julia depends on several external packages; most are automatically downloaded and installed, but are less frequently updated than Julia itself.
+
+* Keep Julia current. Julia is a fast-moving target, and many details of the language are still settling out. Keep the repository up-to-date and rebase work-in-progress frequently to make merges simpler.
+
+* Learn to use [git](https://git-scm.com), the version control system used by GitHub and the Julia project. Try a tutorial such as the one [provided by GitHub](https://try.GitHub.io/levels/1/challenges/1).
+
+* Review discussions on the [Julia Discourse forum](https://discourse.julialang.org).
+
+* For more detailed tips, read the [submission guide](https://github.com/JuliaLang/julia/blob/master/CONTRIBUTING.md#submitting-contributions) below.
+
+* Relax and have fun!
+
 ### Writing tests
 
 There are never enough tests. Track [code coverage at Codecov](https://codecov.io/github/JuliaLang/julia), and help improve it.