Issue 149: Add auto-generation and publication of API JavaDocs

[pmonks]

This PR addresses issue #149, by adding automatic generation and publication of API JavaDocs upon push of a PR to the master branch (note: there may be a better condition to ensure this job only fires when a new version of the library is released / published - I'm not familiar enough with the project's branching / tagging approach to have set that up in this PR).

This PR is necessary but not sufficient to enable API JavaDoc generation and publication - it will also require some configuration of the GitHub project itself by a project owner. Note that in the past I've noticed a one-time "chicken and egg" problem with these setup steps:

• the project configuration changes can't be made until the action has run at least once (in order to create the gh-pages branch that needs to be referenced in that configuration)
• the action fails the first time because the project isn't configured for GitHub pages yet

This is a minor one-time annoyance, but I thought it worth noting as I've been bitten by it in the past and thought something had gone wrong.

[pmonks]

@goneall just a gentle head's up that the "chicken and egg" problem may have reared its ugly head - https://spdx.github.io/Spdx-Java-Library/ is currently returning a 404. From memory manually re-running the GitHub action may fix it, if indeed that's the issue (it's always possible I messed up something in this PR!).

[goneall]

@pmonks

Got it working - had the pages a bit miss-configured, but it is now corrected and looks like it's showing up

[pmonks]

Awesome! Yeah it's a bit fiddly the first time, but after that it should Just Work™️.

Thanks for looking at this so promptly!