Substitutions: use alias for DITA Open Toolkit

- add replace directive in conf.py
- enclose replace directive in triple double quotes in conf.py
- replace DITA Open Toolkit by |dita-ot|, except in titles (because the
  substitution contains an abbr directive)
- reflow some text

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