RFR: JDK-8251210: Link JDK api docs to other versions

[Jonathan Gibbons]

Please review a small change to Docs.gmk, such that the short version string in the upper right corner of the main API docs bundle is linked to a page that links to other versions of the documentation.  While it is not so useful in this release, to be able to access _older_ versions, going forward, this will simplify the ability to get to _newer_ versions when search engines take you to the docs for an older release.

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Commit messages:
 - JDK-8251210: Link JDK api docs to other versions

Changes: https://git.openjdk.java.net/jdk/pull/2854/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2854&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8251210
  Stats: 10 lines in 2 files changed: 9 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2854.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2854/head:pull/2854

PR: https://git.openjdk.java.net/jdk/pull/2854