RFR: JDK-8295914: Add a header to generated HTML files in specs [v2]

[Jonathan Gibbons]

> Please review some updates to the parts of the build system related to building the documentation.

Jonathan Gibbons has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Merge remote-tracking branch 'upstream/master' into 8295914.spec-headers
 - Address review feedback
 - JDK-8295914: Add a header to generated HTML files in specs

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Changes:
  - all: https://git.openjdk.org/jdk/pull/10891/files
  - new: https://git.openjdk.org/jdk/pull/10891/files/c5e567fd..6a1fdfc9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10891&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10891&range=00-01

  Stats: 3212 lines in 138 files changed: 2247 ins; 376 del; 589 mod
  Patch: https://git.openjdk.org/jdk/pull/10891.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10891/head:pull/10891

PR: https://git.openjdk.org/jdk/pull/10891