Docs infrastructure for 7.2.0

All changes necessary to build the release/7.2 branch in docs staging.

Create 7.5 branch in docs-server, including non-7.2 commits
Revert non-7.2 commits in 7.2 branch
Create separate 7.1.x-docs and 7.2 branches in backup and couchbase-cli
Create separate 7.1.x-docs and 7.2 branches in cbas-core and asterixdb
Update assembly for 7.1 and 7.2 branches in cbas-core
Update Jenkins build for 7.1 and 7.2 branches in cbas-core
Create 7.5 branch in docs-analytics
Create 7.5 branch in cb-swagger
Update staging branch on docs-site to include 7.1.x-docs and 7.2 branches
Update master branch on docs-site to include 7.1.x-docs branches