docs: Rewrite the contributing guide

Expanded the introduction and possible areas of contribution. Also
improved structure and added more links to communication channels.

Removed the description of how patches can be submitted, in favour
of a more prominent link to welcome.gnome.org, which provides this
information in much more depth.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/855>

CONTRIBUTING.md

@@ -1,7 +1,32 @@
 # Contributing
 
-When contributing to the development of GNOME Calendar, please first discuss the change you wish to
+Thank you for considering contributing to the GNOME Calendar project! These guidelines are meant
-make via issue, email, or any other method with the maintainers before making a change.
+for new contributors, regardless of their level of proficiency; following them allows other
+contributors to the GNOME Calendar project to more effectively evaluate your contribution, and
+provide prompt feedback to you. Additionally, by following these guidelines you clearly communicate
+that you respect the time and effort that the people working on GNOME Calendar put into managing
+the project.
+
+GNOME Calendar is free software, and it would not exist without contributions from the free
+software community. There are many things that we value:
+
+- Issue triages
+- Bug reports
+- Feature requests
+- User experience reports
+- Translations
+- User interface designs
+- Documentation improvements
+- Patch reviews
+- Test additions
+- Bug fixes
+- Performance enhancements 
+- Source code cleanups
+- Feature implementations
+
+If you are interested in helping us with any of these categories (or anything else that we did not
+think about when writing this), then please read on to learn more about our guidelines and
+processes :)
 
 ## Code of Conduct
 
@@ -23,34 +48,30 @@ to avoid them in favor of using [existing documentation](https://developer.gnome
 [chats and forums](https://welcome.gnome.org). Since AI generated information is frequently
 misleading or false, we cannot supply support on anything referencing AI output.
 
-## Pull Request Process
+## Communication Channels
 
-1. Ensure your code compiles and doesn't break anything. Run `meson test -C <builddir>` before creating
+- Technical discussions happen in the issue tracker and on proposed patches directly, which are both
-   the pull request.
+  hosted on [GNOME's Git forge][forge].
-2. If you're adding new API, it must be properly documented.
-3. The commit message is formatted as follows:
 
-   ```plain
+- Day-to-day conversations, coordination and socialising with other contributors happens primarily
-   component: <summary>
+  in our group chat on [Matrix][matrix].
 
-   A paragraph explaining the problem and its context.
+- Support for users is provided on [GNOME Discourse][discourse], in the "Applications" category,
+  using the "calendar" tag.
 
-   Another one explaining how you solved that.
+# Next Steps
 
-   <link to the issue>
+- Check out [GNOME Calendar on welcome.gnome.org][welcome], which contains concrete steps and
-   ```
+  instructions on how to contribute in a variety of different ways.
 
-4. You may merge the pull request in once you have the sign-off of the maintainers, or if you
+- Join our group chat on [Matrix][matrix]! We are happy to assist you with your contribution, as
-   do not have permission to do that, you may request the second reviewer to merge it for you.
+  well as having a nice chat about all things Calendar :)
 
-## About GNOME Calendar  
+- If you intend to work on the source code of GNOME Calendar directly, you may want to check out our
+  [Hacking Guide][hacking], which explains how to work with the code base in more depth.
 
-Learn more about GNOME Calendar by reading our [HACKING.md](HACKING.md) file.
+[forge]: https://gitlab.gnome.org/GNOME/gnome-calendar/
-
+[welcome]: https://welcome.gnome.org/en-GB/app/Calendar/
-### Attribution
+[matrix]: https://matrix.to/#/#gnome-calendar:gnome.org/
-
+[discourse]: https://discourse.gnome.org/tags/c/applications/7/calendar/22
-This Code of Conduct is adapted from the [Contributor Covenant][homepage], version 1.4,
+[hacking]: ./HACKING.md
-available at [http://contributor-covenant.org/version/1/4][version]
-
-[homepage]: http://contributor-covenant.org
-[version]: http://contributor-covenant.org/version/1/4/