[PATCH] HACKING GUIDE: update and relocate advice on how to write log message for branches.

[[[

Rework branch and log message documentation. Tidy HTML.

* community-guide/conventions.part.html

   (log-messages): Rework relocated branch log message documentation
     moved from community-guide/general.part.html#lightweight-branches.
     Remove mention of 'CIA' and substitute with ASFBot and update
     link. Adjust paragraph containing link to 'partial-commit-access'
     to columns limit.

* community-guide/general.part.html

   (branch-creation-and-management): Add link to log message section.

   (lightweight-branches): Relocate documentation pertaining to log
     messages to community-guide/conventions.part.html#log-messages.

]]]