Improve docs authoring experience

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)

Start by reviewing the Antora Atlas extension and the partial build extension linked in the issue, then examine the current documentation build workflow. Define how UI bundle caching and partial builds would affect the two-repository setup; done means a documented, working authoring workflow with shorter build times.