ICU-23248 Generate more modern javadoc for site

[mihnita]

This was discussed by email in the ICU team ("Updating javadoc look and feel?", May 15).

It generates a more modern javadoc:

• left panel (navigation bar?) with sections, TOC, and filtering
  Compare MessageFormat current vs MessageFormat new
• more mobile friendly (try making the window narrower in namespace view / class view)
• theme support (at the top, after OVERVIEW, TREE, INDEX ... HELP there is a sun / moon icon)

---

I tested the changes by running the action in my repo.

See the result of the run here: https://github.com/mihnita/icu/actions/runs/32469385369
The artifacts (icu4j-binaries and icu4j-fulljavadoc) are at the bottom of the action.
They are also uploaded in the (fake) release in my repo: https://github.com/mihnita/icu/releases/tag/release-79.1

Then I downloaded the artifacts attached to the action and diff-ed them against the ones in the release (identical).

Finally, I extracted the icu4j-79.0.1-fulljavadoc.jar and posted it to my web site: https://mihai-nita.net/tmp/javadoc26/
Then I navigated it randomly, did some searches, etc. All good, from all I can see.

Checklist

• Required: Issue filed: ICU-23248
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-NNNNN Fix xyz"
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-NNNNN Fix xyz"
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable
• Approver: Feel free to merge on my behalf

[mihnita]

Note: the 4 warnings in the [action run] "Annotations" are bogus.

They are caused by javadoc running on main/core not finding classes in main/charset and main/translit.
Everything works when the various javadoc(s) get merged into the final one for the site.

[WARNING] Javadoc Warnings
Warning: [WARNING] /home/runner/work/icu/icu/icu4j/main/core/src/main/java/com/ibm/icu/util/ULocale.java:96: warning: reference not found: com.ibm.icu.text.Collator
[WARNING] * com.ibm.icu.text.UFormat}, {@link com.ibm.icu.text.BreakIterator}, {@link
[WARNING] ^

See it here: https://mihai-nita.net/tmp/javadoc26/com/ibm/icu/util/ULocale.html
Search for Collator and click on it.

Warning: [WARNING] /home/runner/work/icu/icu/icu4j/main/core/src/main/java/com/ibm/icu/text/UnicodeFilter.java:14: warning: reference not found: com.ibm.icu.text.Transliterator
[WARNING] * {@link com.ibm.icu.text.Transliterator} to only process selected characters through a
[WARNING] ^
[WARNING] 2 warnings

See it here: https://mihai-nita.net/tmp/javadoc26/com/ibm/icu/text/UnicodeFilter.html
Search for Transliterator and click on it.