doc/cephfs: Improve formatting in mantle.rst

doc/cephfs: Improve formatting in mantle.rst

Use ordered lists instead of hardcoded list item number paragraphs.

Indent list item contents correctly so that a text block is not
rendered inside a previous preformatted block.
Also fix indentation of one preformatted block inside a list item to be
at the same amount of indent as other such blocks.

Use inline preformatted for commands, method/function names etc. instead
of italic/MD-style inline preformatted.

Signed-off-by: Ville Ojamo <14869000+bluikko@users.noreply.github.com>