libpqxx 7.9.1: Re-do documentation

Documentation on ReadTheDocs was broken as of 7.9.0 (see #802). I rebuilt the whole thing from scratch — both local documentation builds and the ones on readthedocs. I think the new build is actually simpler than it was, which I hope will make it less sensitive to changes on ReadTheDocs or in dependencies.

What else changed?

• Fix bad conversion of array of empty strings to string. (#816)
• Move [[likely]] feature check back to compile time, to speed up configure.
• Support [[assume(...)]].
• Fix throw_null_conversion compile error when converting enums. (#807)
• Fix memory leak when failing to connect.
• Improve introductory docs for connections and transactions.
• No more autodetection of doxygen etc. Run it, or don't.
• Docs build now disabled by default; use --enable-documentation to change.