manual/getting started: rework 'Community resources' section

This patch reworks the section 'Community resources' as follows:
- use a labeled list instead of subsections, as the content of each section
  is so small that it does not deserve a real section.
  Related to this, the anchors for each of these subsections are removed, as
  referencing such anchors that do not correspond to a section boundary are
  not displayed nicely.
- move the part about applying patches from patchwork to the Contributing
  section in the Developer Guide
- reword some parts of the text
- add link texts to some links, instead of showing the URL directly, to make
  the HTML / PDF display nicer.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>