web: In README, CSS generated for ffmpeg repo not merged anymore due to copyright requirements
web/documentation: Simplify Components Documentation links
web: In the nav, Developers links to developer.html + new Contribute link
web/consulting: fix a few typos

Signed-off-by: Lou Logan <lou@lrcd.com>
web/contact: replace MPlayer guidelines link

Replace with FFmpeg Developer Documentation and mention git-howto.

Signed-off-by: Lou Logan <lou@lrcd.com>
web/contact: make valid HTML

Signed-off-by: Lou Logan <lou@lrcd.com>
web/download: Add FFmpeg 2.4
web/documentation: add link to 2.4 doxy