docs: server: Clarify misaka in markup section
docs: server: Clarify misaka in markup section

Add note about misaka 1.0/2.0 discrepancy.

Align example `[markup]` conf block inside docs with the
defaults in `util/html.py`.

Fix formatting issue, code instead of link.

posativ/isso,posativ/isso,jelmer/isso,jelmer/isso,posativ/isso,posativ/isso,jelmer/isso,jelmer/isso