Fix formatting, remove dead links/outdated docs

Fix formatting, remove dead links/outdated docs

- Removes the outdated scenario documentation
  These docs included links to images that no longer exist
- Removes the broken links to apex, sfc, clover, doctor, and ipv6
  These no longer work as the infersphinx linking has been removedA
- Removes links to missing documents
- Fixes a minor issues with the index header
- Updates the docs-linkcheck tox environment with the correct path to
  requirements.txt
- Replaces the IRC link to Slack

Signed-off-by: Trevor Bramwell <tbramwell@linuxfoundation.org>