feat: let Antora manage version aliases (latest & next) (#520)

Changes
- Configure Antora to handle latest/next
- Manage the versions pass to DocSearch: create a custom Antora
extension to pass new information to the bonita-documentation-theme
- Remove netlify redirects that are now managed by Antora (latest and
next)
- Update the "documentation test resources" to ensure we 
- GH Action: 
- the preview url for the PR comment now uses latest to match the url in
the surge preview
- fix an error in the shared action that prevented to compute the
preview url for the PR comment
- documentation: add a paragraph about how Antora caches the git
repositories and the ui-bundle