doc: README: Rework Resources section

With close to 20 different links, the Resources section was not really
easy to navigate, defeating the purpose of providing quick access to
information. Reworked the section to break it down into sub sections,
and reworded the links to make them self explanatory.

Each sub-section has been re-ordered so that most important links appear
first.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>