docs/manual: always point to the correct license file

docs/manual: always point to the correct license file

The manual is GPL-2, and points to the COPYING file in the repository.
When we do a rendering of the manual for a specific version, that URL
is currently always poitning to the latest version of the COPYING file.

If we ever have to change the content of that file (e.g. to add a new
exception, more clarifications, a license change, or whatever), then
an old manual would point to that newer version, which would then be
incorrect.

Include the sha1 of the commit in the URL, so that the manual always
point to the tree at the time the manual was rendered, not the time
it is consulted. Contrary to the informative text above, use the full
sha1, not the shortened one.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Cc: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>