RFR: JDK-8309471: Limit key characters in static index pages

[Hannes Wallnöfer]

Backport of openjdk/jdk#14312 to limit first character keys in the static JavaDoc index pages to valid Java identifier start characters.

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Commit messages:
 - Backport 21f6d83358458c2e5a6cc1bc19a6a33b3262f990

Changes: https://git.openjdk.org/jdk21/pull/30/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21&pr=30&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309471
  Stats: 103 lines in 3 files changed: 87 ins; 0 del; 16 mod
  Patch: https://git.openjdk.org/jdk21/pull/30.diff
  Fetch: git fetch https://git.openjdk.org/jdk21.git pull/30/head:pull/30

PR: https://git.openjdk.org/jdk21/pull/30