Documentation/security-bugs: move from admin-guide/ to process/

Jiri Kosina, Jonathan Corbet, and Willy Tarreau all expressed a desire
to move this document under process/.

Create a new section for security issues in the index and group it with
embargoed-hardware-issues.

I'm doing this at the start of the series to make all the subsequent
changes show up in 'git blame'.

Existing references were updated using:

  git grep -l security-bugs ':!Documentation/translations/' | xargs sed -i 's|admin-guide/security-bugs|process/security-bugs|g'
  git grep -l security-bugs Documentation/translations/ | xargs sed -i 's|Documentation/admin-guide/security-bugs|Documentation/process/security-bugs|g'
  git grep -l security-bugs Documentation/translations/ | xargs sed -i '/Original:/s|\.\./admin-guide/security-bugs|\.\./process/security-bugs|g'

Notably, the page is not moved in the translations (due to my lack of
knowledge of these languages), but the translations have been updated
to point to the new location of th...