website: make a symlink for the manual at the root of the website

This will give us a shorter URL, that we can more easily refer to in the
documetation itself, in help texts, on IRC...

[Peter: Use buildroot.org everywhere]
Suggested-by: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>