docs: Rename `contributing/gsoc-ideas` -> `contributing/gsoc`.

This page contains a lot of other material related to GSoC than
just project ideas.
We would also want to add a redirect from the old URL to the new
one from the RTD admin page.

CONTRIBUTING.md

@@ -357,7 +357,7 @@ community and fixing issues in Zulip now. Having good contributions
 and building a reputation for doing good work is the best way to have
 a strong application.
 
-Our [GSoC project ideas page][gsoc-guide] has lots more details on how
+Our [GSoC program page][gsoc-guide] has lots more details on how
 Zulip does GSoC, as well as project ideas. Note, however, that the project idea
 list is maintained only during the GSoC application period, so if
 you're looking at some other time of year, the project list is likely

docs/contributing/index.md

@@ -12,6 +12,6 @@ zulipbot-usage
 accessibility
 bug-reports
 ../code-of-conduct
-gsoc-ideas
+gsoc
 summer-with-zulip
 ```