Auto-format markdown

The markdown linter from the sphinx starter pack isn't doing much.
We can consider https://pypi.org/project/rumdl/

Related:
- https://github.com/canonical/sphinx-docs-starter-pack/issues/411

> This one doesn't follow the formatting of your other docs (you're using line limits - others are paragraph / essay formatted)
>
> You could put this in your `review-docs` file :)
>
> But also, it doesn't _need_ to be standard paragraph formatted - if you prefer something else, then your team can use that. But it should be consistent throughout your docs if you decide to change it (and ideally have a `make` command or something formatting).
>
> My personal preference (in order) is
> 1. Paragraph formatted
> 2. Line breaks at sentence termination
> 3. Line lengths / line character limits
>
> And my preferences are mostly based on what looks more logical to a contributor outside the team who may or may not be that familiar with seeing/contributing to software docs, and that a modern code editor would line wrap.
>
> But really it's up to your team. You can live your glorious line length limiting life if that's what you want

_Originally posted by @YanisaHS in [#284](https://github.com/canonical/observability-stack/pull/284/changes#r3114007681)_