Add enhancements to docs build

[acozine]

Two enhancements/options would really improve local builds:

• an option for building with no collections
• an option for building with a selected list of collections

These could be two separate flags (--base-only and --build-collections coll1 coll2) or they could be two different options on the same flag (--collections none or --collections coll1 coll2).

[abadger]

I assume the entrypoint for this is going to be something along the lines of make webdocs COLLECTIONS= and make web_docs COLLECTIONS=community.crypto,community.network ? I think the first request is already done but not available at the make webdocs level. The second one is probably a part of #55 but we'll need to remember to hook that up to make webdocs as well.