Build Eclipse Che documentation with Antora.

Rationale:

• Currently built with Jekyll. Theme and jekyll version are very old and unmaintained. Security alerts on jekyll components https:https://github.com/eclipse/che-docs/pull/904 https://github.com/eclipse/che-docs/pull/896
• Website will be easier to maintain with Antora.
• ffloreth did some preliminary analysis which shows there is not too much work to achieve the goal: https://github.com/themr0c/che-docs/blob/antora/README-antora.adoc

DONE: These requirements are met in https://github.com/themr0c/che-docs/tree/antora

1. Usage documentation: README-antora.adoc
2. Build script: run_antora.sh
3. Playbook definition including attributes: antora-playbook.yml.
4. Component definition: antora.yml
5. Modules organization: modules/ with symbolik links.
6. Handle UI: header and footer (supplemental-ui).
7. Handle navigation: modules/<module-name>/nav.adoc.

TODO

1. Replace all internal links using link: by proper xref:. -> Need to apply substitutions when we switch publication platform.
2. Handle images. (no support for subdirectories). -> Need to move files and apply substitutions when we switch publication platform.
3. Handle jekyll headers. (Remove them?). -> Need to apply substitutions when we switch publication platform.
4. Handle redirections from older publish URL. -> Need list of URLs to redirect. Then create an .htaccess in supplemental-ui.
5. Handle publication of website. -> Modify Jenkins job
6. Handle publication of WAR to embed in Che. -> just a copy. The WAR is built by Che. (verify why we have this war option in current build script).