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.

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