Fix Antora pipeline

[julgus]

The Antora build pipeline used for rendering the documentation is malfunctioning and currently only generating docs for versions prior to 1.1.4.

[julgus]

For some unknown reason, building Antora with sources from GitHub prohibits the release of docs from all tags. For now, only versions prior to 1.1.4 are picked up. However since the local build works well I have updated the release_docs.sh script to use the local sources instead. This has no impact on the rendered docs. You just need to make sure that your local repo
includes everything you want to release.

I have also updated the outdated LUNR dependency that facilitates the search functionality and updated the JS and CSS templates accordingly.