It would be good link JDK API docs to other versions. Cross-linking individual pages would be extremely difficult, but adding a link in the top navbar to a page listing the docs for supported JDK versions is much more reasonable.

For new docs going forward, this can be done in the build. For docs that have already been published, we would have to update existing docs, perhaps by directly updating the HTML in place.

For new docs going forward, the proposal is modify the text in the upper right corner, which currently identified the current JDK version by adding a link labeled "Other Versions" that links to a URL specified in the build, which it is expected would default to a page on docs.oracle.com.

The text is currently of the form

    Java SE 11 & JDK 11

or for an EA release

    Java SE 15 & JDK 15
    DRAFT 15-ea+34-1554

The proposal is to replace these two forms as follows:

    Java SE 11 & JDK 11
    Other Versions

or for an EA release

    Java SE 15 & JDK 15 (DRAFT 15-ea+34-1554)
    Other Versions

in both cases, the words "Other Versions" is a link to an appropriate URL. I suggest that the text should be right-justified: I cannot easily demo that in this description.

In all these cases, the content of the upper-right corner is provided by the build, in make/Docs.gmk, in lines building up to this one
http://hg.openjdk.java.net/jdk/jdk/file/bc6a10bc5090/make/Docs.gmk#l311
Therefore, this change can be done entirely in make/Docs.gmk, with no change required to the javadoc tool.

A suitable target page for the "Other Versions" link might be:
   https://docs.oracle.com/en/java/javase/index.html
although it doesn't link to EA versions.