Move documentation documentation to documentation

[samsrabin]

(Sorry for the too-clever title, but I couldn't resist.)

We currently have the Directions for editing CLM documentation on github and sphinx page on the wiki, but it's easy for this to fall out of date. It would be easier to keep our documentation instructions synced with the latest code if we moved those instructions to the actual User's Guide.

[samsrabin]

Could maybe write doc-builder documentation in Markdown and embed that in the User's Guide page using mdinclude; see #2425 for an example.

[samsrabin]

This is done.