Migrate site infrastructure to Antora

[vy]

apache/logging-log4j2#2427 migrated Log4j 2 to Antora. apache/logging-log4j2#2443 cloned this work for Log4j 3. This PR clones the very same work to logging-parent to streamline it for all projects extending from it. A short summary of deliverables are shared below.

Deliverables

• Website is migrated to Antora
• Auto-generated release notes is re-worked to avoid cluttering the Git history. Now it only runs during the pre-site phase and outputs to target/generated-site instead. That is, compiled src/changelog will not be dumped to src/site anymore.
• Auto-generated constants.adoc is directed to target/generated-site too (similar to release notes explained above)
• No web server is needed to view the generated website anymore, one can instead simply open target/site in a browser

Implementation notes

Due to backward incompatible changes, the major version is bumped to 11.

Review kit

1. Start with pom.xml changes
2. Review changes to the website, i.e., src/site/antora/modules/ROOT
3. Review changes to .github

Future work

Once this feature gets released, pom.xml of Log4j 2 and 3 can be trimmed significantly.

[ppkarwasz]

LGTM

[ppkarwasz]

Since all this PR does is modifying Github Actions can you stage the website after merging this PR to confirm that it works as required?

[vy]

Since all this PR does is modifying Github Actions can you stage the website after merging this PR to confirm that it works as required?

I will do that in my follow up PR, where I will implement automatic website publication. 😉