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>

docs/pxar/description.rst

@@ -47,7 +47,7 @@ by running:
 Be aware that the shell itself will try to expand all of the glob patterns before
 invoking ``pxar``.
 In order to avoid this, all globs have to be quoted correctly.
- 
+
 It is possible to pass the ``--exclude`` parameter multiple times, in order to
 match more than one pattern. This allows you to use more complex
 file exclusion/inclusion behavior. However, it is recommended to use