[skip changelog] Update contributions guidelines and PR template to add labeling and increase focus on breaking changes

Author: rsora

Diff for: .github/PULL_REQUEST_TEMPLATE.md

@@ -16,7 +16,7 @@
 * **What is the new behavior?**
 <!-- if this is a feature change -->
 
-- **Does this PR introduce a breaking change?**
+- **Does this PR introduce a breaking change, and is [labeled accordingly](https://arduino.github.io/arduino-cli/latest/CONTRIBUTING/#pull-requests)?**
 <!-- What changes might users need to make in their workflow or application due to this PR? -->
 
 * **Other information**:

Diff for: docs/CONTRIBUTING.md

@@ -54,6 +54,8 @@ submitting a PR:
 - PR titles indirectly become part of the CHANGELOG so it's crucial to provide a good record of **what** change is being
   made in the title; **why** it was made will go in the PR description, along with a link to a GitHub issue if it
   exists.
+- Please start the commit message and PR title with the string **[breaking]** if the PR contains a breaking change.
+  A breaking change is a change that forces users to make changes in their arduino-cli workflow or client application.
 - Write tests for the code you wrote.
 - Open your PR against the `master` branch.
 - Maintain **clean commit history** and use **meaningful commit messages**. PRs with messy commit history are difficult