[Docs] Reorg Compatibility Matrix

[lukas-vlcek]

The Compatibility Matrix in the main branch lists releases for both the recent versions (2.x) and previous releases (1.3.x). When a maintenance release is made for 1.3.x version we need to update lists in both branches (main and v1.3). This is unnecessary and error-prone (especially when OpenSearch 3.x will be released which can bring another branch to maintain...).

See here for an example: #217

The Proposal

Maintain lists of releases in related branches only.
This means that in the main branch we will list only releases of the recent version and for all other versions users will have to switch to respective branch.