Major revamp of the ament_cmake documentation. (#3812)

* Major revamp of the ament_cmake documentation.

Rearrange the document to more closely follow the order
a typical CMakeLists.txt is arranged in.  While doing that,
update the documentation to match our current best practices,
and add in additional advice for building/installing
executables as opposed to libraries.

Signed-off-by: Chris Lalancette <[email protected]>
Co-authored-by: Ryan Friedman <[email protected]>