Fix gitignore to not ignore files that are required (#7538)

It is apparently possible to add files to the git index, even if they
are part of the gitignore (see e.g.
https://stackoverflow.com/questions/45400361/why-is-gitignore-not-ignoring-my-files,
even though it's strange that the gitignore entries existed before the
files were added, i wouldn't know how to get them added in that case). I
ran
```
git rm -r --cached .
```
then change the gitignore not actually ignore those files with the
exception of
`crates/ruff_cli/resources/test/fixtures/cache_mutable/source.py`, which
is actually a generated file.

docs/.gitignore

@@ -1,9 +1,5 @@
-*
-!assets
-!configuration.md
-!editor-integrations.md
-!faq.md
-!installation.md
-!requirements.txt
-!tutorial.md
-!usage.md
+contributing.md
+index.md
+rules.md
+rules/
+settings.md