Add guide for GSoC contributors

CONTRIBUTING.md

@@ -1,5 +1,7 @@
 # Contributing
 
+For candidates interested in participating in the Google Summer of Code (GSoC), checkout Mesa’s [GSoC guide](https://github.com/projectmesa/mesa/blob/main/docs/GSoC.md).
+
 As an open source project, Mesa welcomes contributions of many forms, and from beginners to experts. If you are
 curious or just want to see what is happening, we post our development session agendas
 and development session notes on [Mesa discussions]. We also have a threaded discussion forum on [Matrix]

README.md

@@ -63,6 +63,7 @@ pip install -U -e git+https://github.com/YOUR_FORK/mesa@YOUR_BRANCH#egg=mesa
 For resources or help on using Mesa, check out the following:
 
 -   [Getting Started](https://mesa.readthedocs.io/stable/getting_started.html) (A collection of tutorials that will walk you through a basic model.)
+-   [GSoC at Mesa — Candidates Guide](https://github.com/projectmesa/mesa/blob/main/docs/GSoC.md) (For candidates interested in participating in the Google Summer of Code at Mesa)
 -   [Mesa Examples](https://mesa.readthedocs.io/stable/examples.html) (A repository of seminal ABMs that are part of the Mesa[rec] install and use the most current Mesa release)
 -   [Docs](http://mesa.readthedocs.org/) (Mesa's documentation, API and useful snippets)
     -   [Development version docs](https://mesa.readthedocs.io/latest/) (the latest version docs if you're using a pre-release Mesa version)

docs/index.md

@@ -112,6 +112,7 @@ API Documentation <apis/api_main>
 - {ref}`search`
 
 [contributors guide]: https://github.com/projectmesa/mesa/blob/main/CONTRIBUTING.md
+[GSoC at Mesa — Candidates Guide]: GSoC.md
 [github repository]: https://github.com/projectmesa/mesa/
 [github discussions]: https://github.com/projectmesa/mesa/discussions
 [issue tracker]: https://github.com/projectmesa/mesa/issues