* add api user guides in doc
* update user-guides api page
* fix typo: user guides -> user guide
* move api-usage page under advanced category
* flesh out API usage docs
* Build on work by @tungsheng
* Address issues raised in #4037, #3673, and #4243
* Close#4247
Signed-off-by: Steve Traugott <stevegt@t7a.org>
