doc updates, normalize roundup => Roundup, add links, typo fixes

In Tracker in a Nutshell section
  Document interfaces.py in table
  Add links for auditors/reactors to detectors section in table
  Add links for actions in table
  Also mark optional components

Change roundup to canonical Roundup.

Fix unordered lists that were indented causing a list inside a
blockquote. UL lists are not unindented.

Move internal hyperlink references above the section heading they
refer to. Links were jumping after the heading which disorients the
user.

Change reference to never deleting 'users' class to be 'user' class.

fix typos

Remove uneeded trailing whitespace.

Author: rouilj