<i18n dev> RFR: 8365175: Replace Unicode extension anchor elements with link tag

[Justin Lu]

This PR is a cleanup change which replaces anchor element references of the Locale `def_locale_extension` section with the Javadoc link tag. This is a doc only change and is a result of the discussion in the PR of https://github.com/openjdk/jdk/pull/26683.

This includes one additional conversion of the `legacy_language_codes` section to get rid of all Locale.html anchor element references.

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Commit messages:
 - init

Changes: https://git.openjdk.org/jdk/pull/26996/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26996&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365175
  Stats: 36 lines in 12 files changed: 0 ins; 0 del; 36 mod
  Patch: https://git.openjdk.org/jdk/pull/26996.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26996/head:pull/26996

PR: https://git.openjdk.org/jdk/pull/26996