Consider adding a default page for missing technical reference docs

[glopesdev]

Tight integration between editor and the docs website requires handling various edge cases such as missing entries in the table of contents, packages with no references to doc URLs, etc. Currently this is being handled by error messages inside the editor itself.

It might be nice however to have a default URL for this situation which could be called from the editor. Having a webpage for this would make it easier to update with suggestions on how to proceed such as up-to-date links to discussions, conceptual docs, etc.