RFR: 8351881: Tidy complains about missing "alt" attribute

[Nizar Benalla]

Please review this change to add missing "alt" attributes to the left and right pointing angles used in the sidebar hide/show buttons and prevent `jdkCheckHtml.java` from failing.

TIA.

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Commit messages:
 - add missing alt attribute

Changes: https://git.openjdk.org/jdk/pull/24016/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24016&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8351881
  Stats: 4 lines in 1 file changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/24016.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24016/head:pull/24016

PR: https://git.openjdk.org/jdk/pull/24016