docs: Move GSoC ideas to contributing/, and index.

This better fits the organization of our files; I would have fixed
this last year, but it's not easy to update the links from Google's
site.

CONTRIBUTING.md

@@ -297,7 +297,7 @@ same as with GSoC, and it has no separate application process; your
 GSoC application is your ZSoC application.  If we'd like to select you
 for ZSoC, we'll contact you when the GSoC results are announced.
 
-[gsoc-guide]: https://zulip.readthedocs.io/en/latest/overview/gsoc-ideas.html
+[gsoc-guide]: https://zulip.readthedocs.io/en/latest/contributing/gsoc-ideas.html
 [gsoc-faq]: https://developers.google.com/open-source/gsoc/faq
 
 ## Zulip outreach

docs/contributing/gsoc-ideas.md

@@ -1,7 +1,3 @@
-```eval_rst
-:orphan:
-```
-
 # Google Summer of Code
 
 ## About us

docs/contributing/index.rst

@@ -13,4 +13,5 @@ Code contribution guide
    accessibility
    bug-reports
    ../code-of-conduct
+   gsoc-ideas
    summer-with-zulip