Use See Also directive for Git sections

- remove substitution for Git
- insert external URL for Git
- insert link anchor for first occurence of "Git" in pages
- add See also sections with cross-reference to main Git page in pages
  mentioning Git but not explicitly about it

Signed-off-by: Olivier Carrère <carrereo@gmail.com>
Acked-by: Angela Cristina <acristina@redaction-technique.org>
Use See Also directive for sed sections

- remove substitution for sed
- add See also sections with cross-reference to main sed page in pages
  mentioning sed but not explicitly about it

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