doc/cephadm: Fix formatting errors and improvements in osd.rst

doc/cephadm: Fix formatting errors and improvements in osd.rst

Use console code-block instead of literal text to avoid highlighting.

Promptify few last CLI commands that were missing prompts.

Remove lone single-item unordered list formatting.

Add missing full stops.

Linkify mentions of other sections.

Use title case consistently in section titles.

Use :confval: consistently instead of literal text.

Use double backticks consistently.

Use "YAML" and "JSON" instead of literal .yaml and .json.

Fix two occurrences of double backticks missing one backtick that lead
to broken rendering.

Use *italic* RST syntax instead of rendered underscores.

Try to improve language in simple sentences.

Remove unnecessary * characters that were rendered in CLI example
commands.

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