Centralize external links

- centralize external links in conf.py
- insert comments in conf.py to separate prolog sections
- replace links in rst files
- replace Oasis DocBook link by link to Github
- remove substitutions from links

Signed-off-by: Olivier Carrère <carrereo@gmail.com>
Acked-by: Angela Cristina <acristina@redaction-technique.org>