Add deprecation notes to deprecated importers

What does this MR do and why?

We have announced deprecations in 15.7 and 15.8:

• Announce restricting gitlab projects imports to... (!108745 - merged)
• Announce deprecation of GitLab.com importer (!108502 - merged)
• Announce deprecation of import bare repos rake ... (!108507 - merged)

This MR adds deprecation warnings to their documentation pages.

Towards Deprecations 16.0 (&9663 - closed).