Consolidate docs for all supported versions into master

Problems with maintaining multiple branches:
- Backporting changes is comparatively harder
- There is no staging area to test all versions simultaneously
- Potentially different versions of gitbook are present in different branches, some of which are outdated and hard to maintain
- Different pipelines are needed to test/deploy each version

Proposed change:
Maintain directories for different versions in `master` itself, using the same gitbook version. Build scripts and pipelines would need to be changed accordingly.

Thoughts, suggestions, comments are welcome.