docs: Fix misordered headings (#18192)

1. Raised the `Indent Guides` heading to level 2, which is completely
unrelated to `Git`.
2. the `Git` heading now only contains `Git Gutter` and `Inline Git
Blame` as subheadings.
3. The `Indent Guides` heading is now located directly after the `Git`
heading.

Release Notes:

- N/A