docs: improve info about contributing to docs

[dwilding]

This PR addresses a couple of issues:

• The "Contribute docs" link on the docs homepage points to https://github.com/canonical/operator/tree/main/docs, with no advice about how to get started.
• The Documentation section in HACKING.md has irrelevant/outdated info and no clear advice about how to get started.

Main changes:

• In HACKING.md, split the Documentation section in two:
  • Contributing documentation - Added a high-level process for how to contribute, removed irrelevant info about Discourse docs and the release process, put the build instructions in a subsection (and updated them), kept the subsection about style, put the info about doc versions in a subsection.
  • Maintaining the documentation - The remaining subsections, unchanged. Primarily intended for the Charm Tech team.
• In HACKING.md, updated the Contributing section to mention the importance of docstrings and link to "Contributing documentation".
• On the docs homepage, changed the "Contribute docs" link so that it opens "Contributing documentation" in GitHub. Also adjusted the tagline of the link ("Get started on GitHub" instead of "The documentation sources on GitHub").

[benhoyt]

Looks good - just a couple of small suggestions.

[tonyandrewmeyer]

Looks good, thanks! A couple of small suggestions and questions.

[tonyandrewmeyer]

These work in the ReST docstrings, but do they work in the Myst-Markdown general documentation? I think there we need to still use admonitions, right? Different markup to Discourse, but the same approach.

[dwilding]

I'll double check this before merging - thanks for raising it

[dwilding]

I've added an example for MyST Markdown in 50665df

[dwilding]

@tonyandrewmeyer I've addressed all your comments, thanks! How does it look now?

[tonyandrewmeyer]

Looks great, thanks!