Substitutions: use alias for DITA XML

- add replace directive in conf.py
- replace DITA variants by |dita|, except in titles (because the substitution
  contains an abbr directive)

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

Use DITA XML instead of DITA.

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

Insert Notes rubric before footnotes.

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

- replace table by glossary-like list
- merge and rewrite paragraphs
- rewrite documentation source formats description
- set figure legend in italics

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