Convert markdown documentation to reStructuredText

## Motivation

Markdown is great but lacks some interesting features:
- Importing source code with `literalInclude`
- Admonition boxes
- etc.

## Proposed solution
Ideally all markdown files in the `docs/` directory could be converted to reStructuredText.

## Additional context
This issue originated from a [comment](https://github.com/haskell/haskell-language-server/pull/3655#discussion_r1229835773).

Start by inventorying the Markdown files under docs/ and read the originating discussion in pull request 3655. Review the documentation setup to understand how reStructuredText is expected to be used. Done means all applicable Markdown files in docs/ have been converted while preserving their content and enabling the proposed literalInclude and admonition features.

Written by the indexing model from the issue text.