Improve some Bootstrap UI docs and function reference

[cpsievert]

Probably best reviewed commit-by-commit

• More clearly documents that fixedRow() and basicPage() are soft-deprecated and remove them from the function reference.
• More clearly document the similarities and differences amongst fluidPage(), fixedPage(), and bootstrapPage() (now on the same page). Also, clarify their connection to fluidRow() and column().
• Groom the function reference and break the UI sections into more sections

TODO:

• Maybe it makes sense to add a note to other UI functions (e.g., wellPanel()) that implicitly assume Bootstrap is on the page?