add quick reference for actions

[ralfbrown]

Start of a "how to" section to point users at modules for accomplishing specific tasks.

[elstoc]

I think this is generally a good idea, but will need some thought before I decide to merge. A few minor points in the meantime...

• I won't merge this until all of the modules it references have been documented
• This maybe belongs in the guides/tutorials section
• Will need to consider structure, especially if we think this will grow in the future. For example, we might want to consider combining a few of these on a single page and perhaps categorise them somehow
• Probably should be linked from the Overview section
• Section headers mostly use lowercase in the user manual, and these sections should too

[ralfbrown]

I expect that a lot of the entries will be like hsl.md, just one sentence with a link to the relevant module. The longer ones like sky replacement might indeed fit better under tutorials, though what I had in mind was a quick reference rather than detailed instructions.

I think we should avoid combining too many things, or people will have trouble finding what they want because it's under some umbrella term in the section's index.

I realized right after pushing that I had automatically capitalized the section headers. Will fix when revising, once the overall direction is decided.

[github-actions]

This pull request has not had any activity in the past 60 days and will be closed in 365 days if not updated. Please verify it has no conflicts with the master branch and rebase if needed. Please add a comment if you need help or give permission to other people to finish your work.