doc/cephadm: small improvements

doc/cephadm: small improvements

Remove "*" around CLI command placeholders that was rendered in literal strings.
Leave only the usual "<placeholder>" string.

Use title case consistently in section titles.
Use the standard style section title levels markup and preceding newlines.
Add links to more information.
Use double backticks consistently for literal strings.
Rewrap only excessively long lines only in certain places to avoid excessive diff.
Improve markup.

Fix word capitalization and minor changes to words.
Use Monitor, Manager, etc. instead of MON, MGR, etc.
Reword to "a command of the following form" where suitable.

Delete spaces at end of lines. Add full stops or colons.

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