This PR migrates best practices from [the Juju docs](https://canonical-juju.readthedocs-hosted.com/en/latest/user/reference/charm/charm-development-best-practices/), where the best practice applies to Charmcraft. It is paired with a [similar PR for ops](https://github.com/canonical/operator/pull/1615). The intention is to standardise best practices so that the docs can be scraped to collect best practices across Ops, Charmcraft, juju, and elsewhere in the charming ecosystem, to provide a comprehensive checklist for charmers. My suggestion is for ReST we use: ``` .. admonition:: Best practice :class: hint Content goes here ``` This provides an admonition box that has the title "Best Practice", but is otherwise styled as a hint (David Wilding is working on putting a custom icon into the starter pack, to replace the ? icon that Furo defaults to). The best practices here were selected in a review by @tmihoc and @dwilding and myself, although all problems with the suggested wording are my fault alone.