Antora 3.1 upgrade - Githubissues

This upgrades Antora to version 3 (3.1.7 atm).

Tasks:

[x] Updating package.json so it has the latest Antora (3.1.7 atm)
[x] Updating package.json to use the correct options for the Antora builds
[x] Re-integrate search into the build process No more using an own generator, but creating a extension listening to an Antora event
[x] Fix search to only run when really required. The formerly used BUILD_SEARCH_INDEX triggered the indexing preperation step
[x] Removig the xref-validator js code as this is now fully covered in the Antora build process, see https://gitlab.com/antora/xref-validator
[x] Removing the xref-validator step from the pipeline
[x] Removing the generator directory as it is empty now
[x] Removing Edit-this-page js code from the generation code as this is now handled by Antora, see https://docs.antora.org/antora/latest/playbook/content-edit-url/#source-view
[x] Remove all old pdf generating supporting scripts in bin/ linter, instructions_makepdf.md, makepdf, manual_config_pdf
[x] Removing the docs-pdf step from the pipeline
[x] Commenting the uploading pdf step in .drone.star
[x] Harmonize directory names for each extension (antora, asciidoc) + setting it correctly in site.yml
[x] Fix asciidoctor-kroki extension registration in site.yml
[x] Force cheerio to use the htmlparser2 engine - 6x more performant This will take much less time in CI to finish the job freeing ressources - for EACH docs build
[x] Upgrade the pipeline to Node 18 as Node 16 is already EOL

Other repo dependent important notes/steps:

[x] All failing xrefs need to be resolved first - no issues
[x] Check for the existence of attachmentsdir as with Antora 3 we can consistently use the family coordinate attachment$.

Backport to 5.2 and 5.1 - Important reminder: fix the version key in antora.yml !!

owncloud / docs-client-desktop