doc/cephadm: Fix errors and small improvements in services/monitoring.rst

doc/cephadm: Fix errors and small improvements in services/monitoring.rst

Add that certmgr is new in version Tentacle.

Fix broken links with formatting rendered as text.

Capitalize consistently in text HTTP, YAML, URL, Alertmanager, Grafana,
centralized logging; Prometheus or use inline preformat with lower case
when referring to the module name.

Use inline preformat consistently for CLI commands within text
paragraphs.

Use title case in section titles.

Trim section title underline syntax.

Use consistently pre-formatted blocks bash prompt, YAML.

Start all block formatting paragraphs at the left margin instead of
indenting.

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