Add a doc entry for setting up an OxCaml project with dune pkg (ocaml#12780)

Closes ocaml#11652

Adds a tutorial for setting up an OxCaml project with Dune package
management.

I contemplated where this should go, and thought this might be a good
place, after all. It shows (1) how to setup an OxCaml project, (2)
servers as a live example of the previous two tutorials.

ocaml#12111 is a more comprehensive tutorial about parametric libraries and
OxCaml, but it doesn't specifically cover the topic of Dune package
management with OxCaml. I recently ported an OxCaml project to build
with Dune package management and the configuration was not as
straightforward as I expected, and thought a tutorial on this topic
might be useful.

This takes some material from https://github.com/gridbugs/hello-oxcaml
(thanks @gridbugs!).

---------

Signed-off-by: Sudha Parimala <sudharg247@gmail.com>

Author: Sudha247

doc/tutorials/dune-package-management/index.md

@@ -25,4 +25,5 @@ dependencies
 pinning
 repos
 locking
+oxcaml
 :::

doc/tutorials/dune-package-management/oxcaml.md

@@ -0,0 +1,104 @@
+# Setting up an OxCaml project
+
+As an example of how to add custom repositories and pin dependencies in projects
+using Dune Package Management, let's look at setting up a basic
+[OxCaml](https://oxcaml.org/) project. OxCaml is a compiler branch with a fast
+moving set of extensions to OCaml. Some tweaks are required to set up a project
+that compiles with OxCaml when using Dune package management.
+
+## Setting up the `dune-workspace`
+
+OxCaml has a custom `opam-repository` that provides compatible dependencies.
+Dune can use packages from this repositories by configuring it in the
+dune-workspace file using the {doc}`repositories stanza
+</reference/dune-workspace/repository>` and {doc}`lock_dir stanza
+</reference/dune-workspace/lock_dir>`.
+
+```{code-block} dune
+(lang dune 3.21)
+
+(pkg enabled)
+
+(repository
+ (name oxcaml)
+ (url git+https://github.com/oxcaml/opam-repository))
+
+(lock_dir
+ (repositories overlay oxcaml upstream))
+```
+
+We would like dune to pick the OxCaml compiler rather than the latest upstream
+one. This can be achieved by manually adding a constraint on the compiler that
+is picked in the {doc}`package stanza </reference/packages>` in
+the dune-project.
+
+```{code-block} dune
+(lang dune 3.21)
+
+(package
+ (name hello-oxcaml)
+ (depends
+  (ocaml-variants
+   (= 5.2.0+ox))))
+```
+
+Let's add a test program that uses OxCaml syntax.
+
+::::{dropdown} `main.ml`
+:icon: file-code
+
+:::{literalinclude} oxcaml/main.ml
+:language: dune
+:::
+
+::::
+
+And to build it, we add a `dune` file.
+
+```{code-block} dune
+(executable
+ (public_name main))
+```
+
+## Building the project
+
+Let's compile and execute the project. Note that when you run `dune pkg lock`
+or `dune build` the very first time, Dune pulls in and builds all the
+dependencies before building the project itself. It is expected to take a
+considerable amount of time.
+
+Note: With autolocking enabled one doesn't need to explicitly run `dune pkg
+lock` to generate a lock directory. However, a known issue,
+[dune#12851](https://github.com/ocaml/dune/issues/12851) currently prevents us
+from using autolocking.
+
+<!-- TODO: Update this after 12851 is fixed. -->
+
+```
+$ dune pkg lock
+Solution for dune.lock
+
+Dependencies common to all supported platforms:
+- conf-autoconf.0.2
+- conf-which.1
+- ocaml.5.2.0
+- ocaml-config.3
+- ocaml-variants.5.2.0+ox
+
+$ dune exec ./main.exe
+...
+43
+```
+We have successfully built the project with the OxCaml compiler and run the executable.
+
+## Developer tools
+
+Note that OxCaml provides its own forks of the developer tools
+`ocaml-lsp-server`, `ocamlformat`, etc. Make sure to include constraints for the
+tools in the `dune-workspace` if you intend to use them for your development
+workflows.
+
+Please see {doc}`/howto/customize-dev-tools-lock-directories` for details about
+configuring your workspace for developer tools. Please be aware that there are
+known issues in the current Dune release that may prevent this functionality
+from working reliably.

doc/tutorials/dune-package-management/oxcaml/main.ml

@@ -0,0 +1,5 @@
+let () =
+  (* The `local_` keyword requires OxCaml *)
+  let local_ i = 42 in
+  let j = i + 1 in
+  Printf.printf "%d\n" j