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

Author: rsora

.github/PULL_REQUEST_TEMPLATE.md

@@ -16,7 +16,8 @@
 * **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
+[titled accordingly](https://arduino.github.io/arduino-cli/latest/CONTRIBUTING/#breaking)?**
 <!-- What changes might users need to make in their workflow or application due to this PR? -->
 
 * **Other information**:

docs/CONTRIBUTING.md

@@ -54,6 +54,10 @@ 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.
+- <a id="breaking"></a>A breaking change is a change that forces users to make changes in their arduino-cli workflow or
+  client application. If the PR contains a breaking change, please start the commit message and PR title with the string
+  **[breaking]**. Don't forget to describe what changes might users need to make in their workflow or application due to
+  this PR
 - 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