Improve docs authoring experience #519

As a dev, writing docs is made unpleasant by a few things in the process, especially when writing docs for an Operator, because multiple repos are involved.

Currently the workflow is:

• checkout both the docs repo and the operator repo
• edit the playbook file to point to the local operator repo checkout
• write the docs in the operator repo and rebuild the docs periodically

Main pain points are:

• the setup is cumbersome
• build times are long

### Tasks
- [ ] Cache UI bundle in the build
- [ ] investigate Atlas/partial build extension

Links:

• Antora Atlas extension: https://gitlab.com/antora/antora-atlas-extension
• partial build extension: https://github.com/spring-io/antora-extensions#partial-build (depends on the Atlas extension)