Build and release process update

[vojtechhabarta]

Currently build is run on Appveyor, which works fine and I still like it, but it needs some update.

The build of main branch is now failing because (in addition to steps done for all builds) it also creates Maven site (documentation) and deploys it to GitHub pages. For the site deploy it uses com.github.github:site-maven-plugin which is now archived (https://github.com/github/maven-plugins) and fails with an error:

Failed to execute goal com.github.github:site-maven-plugin:0.12:site (site) on project typescript-generator: Error creating blob: cannot retry due to server authentication, in streaming mode

I could probably fix it by just replacing it with some alternative but maybe we could change the whole process. Firstly, documentation (site) should be deployed when creating new release (not when main branch is built). Secondly we could migrate the build and site generation to GitHub actions (which are already used for releasing to Maven central and Gradle plugin portal).

@kkuegler, what do you think? Fix current build for now or migrate to GitHub actions?

[vojtechhabarta]

I've moved Maven site generation and deploy from Appveyor build of main branch to release GitHub Action.
Build of main branch on Appveyor is now fixed (site is not tested yet as it is meant only for release).

This unblocks next development, but I expect that sooner or later we will switch build from Appveyor to GitHub Actions anyway.

[kkuegler]

Sorry for the radio silence. Sounds good!

I also like fully switching to GitHub action in the long run, as that is familiar to all GitHub contributors.