* docs changes for voyager 1.8.12

* release notes for voyager 1.8.12

* Fix formatting in airgapped installation instructions for RHEL8 and RHEL9

* Fix punctuation in airgapped installation instructions for RHEL 8/9 and CentOS 8/9

* minor edits

* put content under main tab airgapped, instead in another file

* Fix comment in PL/SQL code to clarify trigger re-enabling

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Co-authored-by: Dwight Hodge <ghodge@yugabyte.com>