Removes the experimental status for the frozen tier / shared_cache searchable snapshots for the 7.13 release.
Also adapts docs that URL repositories are now supported in 7.13 for searchable snapshots.
Changes:
* Refactors the "Getting Started" content down to one page.
* Refactors the README to reduce duplicated content and better mirror
Kibana's.
* Focuses the quick start on time series data, including data streams
and runtime fields.
* Streamlines self-managed install instructions to Docker.
Co-authored-by: debadair <debadair@elastic.co>
