Annex -> LFS migration guide

We're announcing that we're deprecating Annex in 8.17, then removing it in 9.0.

https://gitlab.com/gitlab-org/gitlab-ee/issues/1648

Todos:

• Draft guide
• Review (Axil, Sean P)

To update:

• https://docs.gitlab.com/ee/downgrade_ee_to_ce/README.html
• https://docs.gitlab.com/ee/workflow/git_annex.html
• https://about.gitlab.com/2015/02/17/gitlab-annex-solves-the-problem-of-versioning-large-binaries-with-git/