AxesInDifferentUnits: restore webdoc and tests

Waiting for #702 (closed).

assigned to @j.wuttke

Test run_example(scatter2d/AxesInDifferentUnits) is enabled.

Page https://bornagainproject.org/git-main/ex/result/export/axes-in-different-units/ also exists.

So what is to be restored?

I guess I should update the webdocs. With #702 (closed), the API may still change.

mentioned in commit c0acfc4f

mentioned in merge request !1972 (merged)

mentioned in commit b6635ba6

closed with merge request !1972 (merged)