Welcome page: add content

- remove previous title (now set as HTML title in conf.py)
- set subtitle as first level title
- add figure and text to illustrate functionality and complexity levels of
  different text formats
- remove links to Sphinx, and Creative Commons
- add text about text formats and revision control systems
- add figure source and target files

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