Restructure docs (more first level headings)

Restructure docs (more first level headings)

This removes the "Backup Management" first level heading in the docs,
and either uses the sub headings contained within it as first level
headings, or groups previous sections logically under new headings.

The administration-guide.rst file is also removed. Its contents are
instead separated into various files, that relate to their respective
first level heading.

Signed-off-by: Dylan Whyte <d.whyte@proxmox.com>