Unifying Kibana 9.0.0-rc1 RNs (#2994)

* simplify structure

* revert to 1 unique file without split

* clean ids

* more clean ids

* structure and remove automatic import

* update borealis highlight

* Update docs/en/install-upgrade/release-notes/release-notes-kibana.asciidoc

Co-authored-by: wajihaparvez <wajiha.parvez@elastic.co>

* Update docs/en/install-upgrade/release-notes/release-notes-kibana.asciidoc

Co-authored-by: Thomas Neirynck <thomas@elastic.co>

* Update docs/en/install-upgrade/release-notes/release-notes-kibana.asciidoc

Co-authored-by: Thomas Neirynck <thomas@elastic.co>

* Update docs/en/install-upgrade/release-notes/release-notes-kibana.asciidoc

Co-authored-by: Thomas Neirynck <thomas@elastic.co>

* Update docs/en/install-upgrade/release-notes/release-notes-kibana.asciidoc

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Co-authored-by: wajihaparvez <wajiha.parvez@elastic.co>
Co-authored-by: Thomas Neirynck <thomas@elastic.co>

Author: 3 people