doc: Fix sphinx warnings

doc: Fix sphinx warnings

doc/cephadm/services/snmp-gateway.rst: Don't use double backticks for
links. Makes it a link instead of rendering syntax verbatim.
Also for consistency use single backticks for links instead of a plain
trailing underscore.
Improve language of opening sentence.

doc/dev/cephfs-mirroring.rst: Add missing empty line before preformatted
blocks. No change in rendered docs.

doc/mgr/telemetry.rst: Fix external link syntax. Makes it a link instead
of rendering syntax and pointing to non-existing link.

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