Provide a way to do partial local subproject builds

[djencks]

Subprojects need to be able to check their documentation reasonably quickly without doing a full Antora build. The current approach of having customized playbooks for each subproject doesn't work.

One clear requirement is that local partial builds must be based on the actual camel-website full build.

I have a POC that I will discuss on the dev list.

[djencks]

I've added this to all the active main camel and camel-quarkus branches (except main 2.x, which I presume is not really active) and haven't seen other comments, but I've found it really useful.

As part of #701 I expect to be touching every active branch of every subproject shortly so I will add this local-build setup to the remaining subprojects.

[djencks]

I'm planning to document how to use this on https://camel.apache.org/manual/improving-the-documentation.html, and replace the subproject docs/README.* with a pointer to that plus any subproject specific instructions.

I'm thinking of naming the local instructions README_local_build.adoc to make it clear what they are for.

[djencks]

As part of #731 I've revised the scripts and setup for local builds, and the 32 PRs for all subprojects + updated docs are listed there.

[djencks]

I think this is now complete.

[djencks]

I think this is done.