Welcome page: add content

- insert table showing which technologies the website is about and to which
  format they apply
- insert links to GitHub and internal pages
- insert URLs in conf.py

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

Edit existing "Git" URL in conf.py to link to internal page.

Signed-off-by: Olivier Carrère <carrereo@gmail.com>
Acked-by: Angela Cristina <acristina@redaction-technique.org>
Welcome page: add Git link anchor

Insert link anchor on "Git".

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