doc/install: improve get-packages.rst

Fix some substitutions not working, printing stable-release instead of a
release name.
Fix indentation both rendered and source consistency.
Change tabs to spaces and delete spaces at end of lines.
Add text about Cephadm private repo support instead of claiming it
requires internet. Add link to Cephadm deployment.
Improve capitalization, use prompt for CLI examples.

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