Consolidate docs for all supported versions into master

[akshaydewan]

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• Potentially different versions of gitbook are present in different branches, some of which are outdated and hard to maintain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.