Non-breaking spaces: insert Unicode U+00A0 character in source files

- remove unicode directive substitution from conf.py
- insert Unicode U+00A0 character in source files
- insert hard spaces in sentences in italics
- insert hard spaces in sentences in links anchors
- fix title adornments
- fix tables

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