Separate Disaster Recovery docs from Geo docs

What does this MR do?

• Move Disaster Recovery docs to the administration/disaster_recovery directory to separate Disaster Recovery docs from Geo docs
• Change references to GitLab Geo Disaster Recovery to Disaster Recovery
• Change references to GitLab Geo to Geo
• Change inconsistent use of failover, fail over and fail-over to failover

Moving docs to a new location?

See the guidelines: http://docs.gitlab.com/ce/development/doc_styleguide.html#changing-document-location

• Make sure the old link is not removed and has its contents replaced with a link to the new location.
• Make sure internal links pointing to the document in question are not broken.
• Search and replace any links referring to old docs in GitLab Rails app, specifically under the app/views/ directory.
• Make sure to add redirect_from to the new document if there are any Disqus comments on the old document thread.
• If working on CE, submit an MR to EE with the changes as well.
• Ping one of the technical writers for review.

Links

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/1989