Substitutions: use alias for reStructuredText

- add replace directive in conf.py
- put "reStructuredText" in italics
- replace existing occurences of "reStructuredText" by aliases, except when used
  as link anchors

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