Add javadoc action

[alanocallaghan]

This builds javadoc on main and publishes it to the latest subdir

vX.Y tags also get built and published to a tag subdirectory

The top level index points to stable/index.html, and stable is a symlink pointing to a tag subdirectory. Currently 0.1.4.

I've only published the 0.1.4, I was gonna build the rest but seems pointless for now.

For new tags, the second job in the action updates the stable link to point to the docs for the current tag. Maybe a problem if we ever publish versions out of order (eg we publish 0.6.0, but find a critical bug in 0.5.3 so publish 0.5.4), but we shouldn't do that often, so I think it's fine

Output is here: https://alanocallaghan.github.io/qupath-fxtras

Absolutely not merging this on a Friday at 5pm, so will revisit next week and probably update https://github.com/qupath/javadoc/ as well... or maybe scrap this and try to do a big merged javadoc. Will see

[alanocallaghan]

Much delayed but I think this should work fine, though I also am willing to take responsibility if not