This PR adds some documentation about the minimap to the official docs.

**Please note:** The [current preview release
notes](https://zed.dev/releases/preview/0.187.0) refer to the minimap PR
for configuration options. However, `font_size` and `width` were removed
as settings after some discussion but are still referenced in the PR
description, which might be misleading. On the other hand, some of the
available configuration options are not listed in the PR description. It
might be better to refer to the docs or the default settings in order to
avoid confusion.

Release Notes:

- N/A