doc/cephadm: small improvements to install.rst

doc/cephadm: small improvements to install.rst

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 confval role instead of literal inline for config keys in text.
Improve markup.
Use ref instead of full URL for intradocs link.
Remove unused external link definition.
Fix double prompts in SSH cert auth section.

Fix word capitalization and minor changes to words.
Use Monitor, Manager, etc. instead of MON, MGR, etc.
Fix hyphenation.
Use underscores in config key name.
SSH cert auth uses a signed public, not private, key.

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

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