documentation/conf.py: define a manpage url

By defining the manpages_url we can use the :manpage: role in the
documentation for providing links to manpages. See:
https://www.sphinx-doc.org/en/master/usage/configuration.html#confval-manpages_url

Replace existing manpages links to use this role.

Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
(From yocto-docs rev: 73faf4a4fdd3d536e8af92b49bf067746cd9fa05)

Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
(cherry picked from commit 4e63cd74cd1a330ea5e96bb04243a90f607b2857)
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

documentation/conf.py

@@ -111,6 +111,9 @@ extlinks = {
     'wikipedia': ('https://en.wikipedia.org/wiki/%s', None),
 }
 
+# To be able to use :manpage:`<something>` in the docs.
+manpages_url = 'https://manpages.debian.org/{path}'
+
 # Intersphinx config to use cross reference with BitBake user manual
 intersphinx_mapping = {
     'bitbake': ('https://docs.yoctoproject.org/bitbake/' + bitbake_version, None)