Signed-off-by: NRémy Coutable <remy@rymai.me>

Because we removed rails 4 support, this document is not needed
anymore.

**Why?**

This is a recurring issue that new team members keep running into.

- Split up refs and kubernetes to their own section
- Use only and except in all headings explicitly
- Document the boolean logic of only and except

The strategy name and interval was incorrect, and the link back to the
developer documentation was in the wrong place.

Pool repositories are persisted in the database, and when the DB is
restored, the data need to be restored on disk. This is done by
resetting the state machine and rescheduling the object pool creation.

This is not an exact replica of the state like at the time of the
creation of the backup. However, the data is consistent again.

Dumping isn't required as internally GitLab uses git bundles which
bundle all refs and include all objects in the bundle that they require,
reduplicating as more repositories get backed up. This does require more
data to be stored.

Fixes https://gitlab.com/gitlab-org/gitaly/issues/1355

`-D` is OSX speific and is invalid on Linux, etc

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/55218

Extending from ActiveRecord::Migration is deprecated, but was still used
in a bunch of places.

Ref
https://docs.gitlab.com/ee/development/documentation/styleguide.html#lin
ks

the added note describes code from https://gitlab.com/gitlab-org/gitlab-ee/blob/master/lib/gitlab/import_export/merge_request_parser.rb#L15

Fix missing empty space that makes rendered gitlab documentation page missing list. Different output between gitlab internal .md reader and docs.gitlab.com rendered documentation.

This reverts commit cc735352.

I pushed this commit to master accidentally. Oops!