doc/start: Update and fix get-involved.rst

doc/start: Update and fix get-involved.rst

Remove not existing Planet Ceph, Wiki, Commit List rows.
Update Kernel Client, QA, Community mailing list links to working ones.
Use https instead of http.
Fix Ceph calendar link and split the old contribute guide link to a
separate table row.
Remove not working lists.ceph.com external link definition now that it
is unused.
Sort external link definitions in order of use.
Fix invalid space after a hyphen by rewrapping text.
Update Slack invite link.

Signed-off-by: Ville Ojamo <git2233+ceph@ojamo.eu>