RFR: JDK-8304689: Add hidden option to disable external spec page

[Jonathan Gibbons]

Please review a change to introduce a hidden option to temporarily disable the "External Specifications" page while we add `@spec` tags, and until we have a critical mass of such tags.

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Commit messages:
 - JDK-8304689: Add hidden option to disable external spec page

Changes: https://git.openjdk.org/jdk/pull/13127/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13127&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304689
  Stats: 58 lines in 5 files changed: 47 ins; 0 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/13127.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13127/head:pull/13127

PR: https://git.openjdk.org/jdk/pull/13127