docs: translate remaining .md files to .rst (#2083)

Translated remaining Markdown files from #2010 to reStructuredText.

The contents of each page remain largely unchanged, with the main
exceptions being:
- Dropdowns in [Manage a 12-factor app
charm](https://canonical-charmcraft.readthedocs-hosted.com/en/stable/howto/manage-a-12-factor-app-charm/)
were replaced with tabs.
- Output of `juju status` in
[tutorials](https://canonical-charmcraft.readthedocs-hosted.com/en/stable/tutorial/)
was changed from a bash code directive to a terminal directive.

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Co-authored-by: Michael DuBelko <michael.dubelko@canonical.com>