magic first line tells emacs to use utf8+bom

Add ".. -*- coding: utf-8-with-signature -*-" to the first line of each .rst
file. This tells emacs to treat the file contents as utf-8, and also to prepend
a so-called utf-8 "bom" marker at the beginning of the file. This patch also
prepends those markers to each of those files.
fix warnings from rst2html

Apparently the in-line link syntax with "<>" in them causes these warnings. I
don't know why. I changed them all to a slightly more verbose syntax.

Thanks to Mike Kazantzsev's review comment
(https://github.com/tahoe-lafs/tahoe-lafs/pull/67#commitcomment-4561370), I
moved the links to the end of each section.
update link to Maildir tips (thanks MK_FG)
change more links to footnote-style
Merge pull request #69 from zooko/1948-fix-warnings-from-make-doc-checks-3

1948 fix warnings from make doc checks 3