fix stray <p> tags on community page

The should have been closing </p> to match the other
paragraphs.
community: mention mailing list HTML gotcha

Many people are caught up by this on their first submission.
Let's also point them to gmane as a potential gateway (I'm
not sure how much less painful it is to use gmane, but it's
worth a shot).
community: give bug reporting its own section

We already mention bug-reporting as a function of the
mailing list, but a first-time reader looking for
instructions on reporting a bug may skip over the whole
mailing list section. Let's make it easier for them to find
what they're looking for.
mention bug reporting on the main page

It may not be immediately obvious that the "community" link
is where you need to go to report bugs. Let's add "bug
reporting" to the synopsis.
community: fix mis-matched h1/h2 tags
community: link to sgtatham's bug reporting advice

This isn't perfect and isn't git-specific at all, but it
hopefully at least gives useful reading to people completely
new to reporting bugs. We should probably write a more
git-specific one that addresses problems we see, but that
doesn't yet exist.
Merge pull request #462 from git/peff/easier-bug-reporting

make bug-reporting instructions easier to find