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

[Iris Clark]

On Sat, 6 Mar 2021 02:21:00 GMT, Jonathan Gibbons <jjg at openjdk.org> wrote:

> 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.

Marked as reviewed by iris (Reviewer).

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