RFR: 8309748: Improve host selection in `External Specifications` page [v3]

[Hannes Wallnöfer]

> Please review a change to improve selection by host name in the External Specifications page. After trying a list of checkboxes as used in the API summary pages and finding it not a suitable solution, I found that a select/dropdown menu works best for this page. 
> 
>  - [New external specs page](https://cr.openjdk.org/~hannesw/8309748/api.00/external-specs.html)
>  - [Old external specs page](https://docs.oracle.com/en/java/javase/25/docs/api/external-specs.html) for comparsion
> 
> I also updated `@spec` tags in `java.lang.Character` to point to `www.unicode.org` rather than `unicode.org` to avoid a  `unicode.org` entry in the host list (other `@spec` tags use the `www.unicode.org` host name). The short `unicode.org` URL is still used elsewhere in the `java.lang.Character` class, I can update those links to the canonical host name as well if desired.

Hannes Wallnöfer has updated the pull request incrementally with one additional commit since the last revision:

  Update copyright headers

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/28863/files
  - new: https://git.openjdk.org/jdk/pull/28863/files/fe88df58..b86db58d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28863&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28863&range=01-02

  Stats: 5 lines in 5 files changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/28863.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28863/head:pull/28863

PR: https://git.openjdk.org/jdk/pull/28863