Antora 3.1 upgrade

[mmattel]

References: owncloud/docs-client-branding#90 (Antora 3.1 upgrade)
Fixes: #4853 (Upgrade the documentation framework)
Fixes: #4545 (Antora release upgrade from 2.3.4 to 3.0.x)

This upgrades Antora to version 3 (3.1.7 atm).

Tasks:

• Updating package.json so it has the latest Antora (3.1.7 atm)
• Updating package.json to use the correct options for the Antora builds
• Re-integrate search into the build process
  No more using an own generator, but creating a extension listening to an Antora event
• Fix search to only run when really required. The formerly used BUILD_SEARCH_INDEX triggered the indexing preperation step
• Removig the xref-validator js code as this is now fully covered in the Antora build process,
  see https://gitlab.com/antora/xref-validator
• Removing the xref-validator step from the pipeline
• Removing the generator directory as it is empty now
• 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
• Remove all old pdf generating supporting scripts in bin/
linter, instructions_makepdf.md, makepdf, manual_config_pdf
• Removing the docs-pdf step from the pipeline (not applicapable)
• Commenting the uploading pdf step in .drone.star (not applicapable)
• Harmonize directory names for each extension (antora, asciidoc) + setting it correctly in site.yml
• Fix asciidoctor-kroki extension registration in site.yml
• 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
• Upgrade the pipeline to Node 18 as Node 16 is already EOL

Other repo dependent important notes/steps:

• All failing xrefs need to be resolved first - none as we do not have any modules/components here and all other repos have been prepared already.
• For the docs repo, we can remove BUILD_SEARCH_INDEX from .drone.star as it is no longer used.
  "BUILD_SEARCH_INDEX": ctx.build.branch == deployment_branch, --> remove.
• Need a docs repo upgrade for Node 18 and a comment that one needs to upgrade his local installation!

Upgrade sort order:

• docs-server + branches
• docs-client-desktop + branches
• docs-client-android + branches
• docs-client-ios-app + branches
• docs-branding
• docs-webui
• docs-ocis
• docs-main
• docs