Add Gradle plugin for publishing JavaDoc to GitHub Pages

[ob-stripe]

r? @brandur-stripe @richardm-stripe
cc @stripe/api-libraries

This PR adds a Gradle plugin (org.ajoberstar.git-publish) that adds support for publishing the JavaDoc HTML output to GitHub pages.

The plugin can be used like this:

$ GRGIT_USER=<GitHub API token> ./gradlew gitPublishPush

This command will build the JavaDoc and push the results to the gh-pages branch. I've configured GitHub pages to use the contents of this branch. Apparently GitHub Pages even knows about the stripe.dev domain, so the results are available at https://stripe.dev/stripe-java.

The intent is that we would run this command automatically in the release pipeline, so the JavaDoc stays up to date. We could then link to it from the README, as requested by a user some time ago (#770).

[richardm-stripe]

lgtm

Although, "github pages knows about stripe.dev" seems backwards to me...
stripe.github.io appears to 301 to stripe.dev, do you happen to know the details of how stripe.dev uses github pages as a backend? Or is it mirrored?

[brandur-stripe]

Niceeee!

The intent is that we would run this command automatically in the release pipeline, so the JavaDoc stays up to date.

Also consider adding it as a GitHub Action — their system for secrets management works pretty well.

We could then link to it from the README, as requested by a user some time ago (#770).

Great idea! I always wished we'd had better docs for the built-in doc system in each given language.

[ob-stripe]

Although, "github pages knows about stripe.dev" seems backwards to me...
stripe.github.io appears to 301 to stripe.dev, do you happen to know the details of how stripe.dev uses github pages as a backend? Or is it mirrored?

I'm unsure, but I got the stripe.dev URL directly from GitHub:

[ob-stripe]

Also consider adding it as a GitHub Action — their system for secrets management works pretty well.

I'm still trying to clarify the status of GitHub Actions on our public repos -- it just got reenabled, but I'd like to make sure it won't be disabled again before we start using it for anything important.