Automatically deploy Javadoc to GitHub pages

[msgilligan]

Since PR #242 we are generating aggregate Javadoc and putting it in a GHA artifact. The next step should be to publish the Javadoc to GitHub pages. This should probably only occur for tagged releases. It would also be nice to be able to have the Javadoc for every released version available on-line.

The following two GitHub-provided actions may do what we want:

• https://github.com/actions/upload-pages-artifact
• https://github.com/actions/deploy-pages

I would like to avoid accumulating all the Javadoc on a gh-pages branch in this repository and first glance at those actions did not make it clear to me whether that happens or not. I'm also not sure if those actions will allow us to add new versions while retaining the old. If we have to we could create a separate repo for publishing Javadoc via GitHub pages.

[msgilligan]

I'm thinking we could create two new repos: bitcoinj-javadoc and secp-jdk-javadoc and change the links in the main doc repo to use those paths. (We would copy all the existing Javadoc into the bitcoinj-javadoc repo.) This would make things more modular, stop the bitcoinj.github.io repo from growing too big, keep publishing permissions more granular, etc.