This commit adds a link in the package-overview pages of KDoc and
Javadoc, pointing to the other documentation's own page.
In order for local tests to work, the build has been slightly modified
to align with the directory structure in the distributed documentation
zip (and consequently with that of the deployed documentation website).
Lastly, the javadoc build was fixed to include the `overview.html` again
in the resulting HTML index, since this is where the KDoc link is added.
See gh-28055
Closes gh-31587
a97ecf7a29