Substitutions: use alias for "rédacteur technique"

- add replace directive in conf.py
- add support for Unicode characters in conf.py
- replace "rédacteur technique" by |techwriter|
- use "English" substitution ("techwriter" instead of "redactech") to better
  support translation

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