[Twisted-Python] Documentation: Footnotes [Yay]

[Moshe Zadka]

So, we now have footnote support, and I fixed all documents[1][2] to take
advantage of it. Hopefully, this will make the next version of the book
even better than ever before.

Indexes are still not supported, but should be even easier to add.

[1]
Except pb-cred.html and pb-tour.html, which use footnotes for unfootnotish
stuff, and thus should be revamped by their author.

[2]
See! I told you we have footnote support