Docs infrastructure for 7.6.0

All changes necessary to build the release/7.6 branch in docs staging.

Create 7.6 branch in docs-server, including non-7.2 commits
Create 7.6 branch in docs-devex, including non-7.2 commits
Check for existence of 7.6 branch in backup and couchbase-cli
Update component descriptor for 7.6 branch in backup and couchbase-cli
Check for existence of 7.6 branch in cbas-core and asterixdb
Update assembly for 7.6 branch in cbas-core
Update Jenkins build for 7.6 branch in cbas-core
Create 7.6 branch in docs-analytics
Create 7.6 branch in cb-swagger
Update staging playbook on docs-site to include 7.6 docs