docs: Convert u2f.txt to rST

docs: Convert u2f.txt to rST

Convert the u2f.txt file to rST, and place it in the right place
in our manual layout. The old text didn't fit very well into our
manual style, so the new version ends up looking like a rewrite,
although some of the original text is preserved:

 * the 'building' section of the old file is removed, since we
   generally assume that users have already built QEMU
 * some rather verbose text has been cut back
 * document the passthrough device first, on the assumption
   that's most likely to be of interest to users
 * cut back on the duplication of text between sections
 * format example command lines etc with rST

As it's a short document it seemed simplest to do this all
in one go rather than try to do a minimal syntactic conversion
and then clean up the wording and layout.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-id: 20230421163734.1152076-1-peter.maydell@linaro.org