doc/install: update get-packages.rst

Bring the document up to date so that URLs actually work and other
information is current.
Cephadm installation only listed commands for el8. Instead of expanding
it for other distros/versions, just refer to the cephadm install doc.
Minor punctuation, capitalization, markup improvements.

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