Infra: Don't generate broken source link for topic pages

[hugovk]

To reproduce:

1. Go to a topic index page:
   • https://peps.python.org/topic/
   • https://peps.python.org/topic/packaging/
   • https://peps.python.org/topic/release/
   • https://peps.python.org/topic/typing/
2. Click "Page Source (GitHub)" in the left menu

Actual result:

404 this is not the page you are looking for, because these pages are generated at build time and don't exist in the Git repo.

Expected result:

Either no 404, or no link in the first place.

Fix:

Let's not generate the link.

Preview:

• https://pep-previews--2843.org.readthedocs.build/topic/
• https://pep-previews--2843.org.readthedocs.build/topic/packaging/
• https://pep-previews--2843.org.readthedocs.build/topic/release/
• https://pep-previews--2843.org.readthedocs.build/topic/typing/

[CAM-Gerlach]

There's also @pradyunsg 's proposal to shift the logic to a directive rather than the somewhat hacky source file generation we're using right now, which would also avoid this :)

[JelleZijlstra]

We can switch to the directive logic when the code for that exists, but in the meantime let's fix this bug. Thanks @hugovk!