Extends `docs.gitlab.com` with feature flags table

Purpose

This is meant to solve: gitlab#221057 (closed).

Now

This provides an initial skeleton to:

• sparse checkout feature flags
• add rake task
• convert all feature flags YAML into data template

Next

Write an .md.erb to generate a table of feature flags.

Preview

http://generate-feature-flags-table.178.62.207.141.nip.io/ee/user/feature_flags.html