doc: Improve "Contributing to Zephyr" landing page

The "Contributing to Zephyr" page used to be a simple table of contents
with pointers to sub-pages. This bland page was not very inviting to new
contributors, and provided little to no context as to what to expect in
each sub-section. This commit improves the landing page by implenmenting
the following changes:

- Remove auto toctree and replace with manually curated entries that
  provide more context about each sub-section.
- Add a section about the Zephyr Contributor Badge.
- Add a section about getting help, with links to Discord and dev
  mailing list.

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