Retrocatively generate warning for outdated docs

[pfitzseb]

Companion to JuliaDocs/Documenter.jl#1577.

This uses the default light theme and a bunch of inline elements to make sure it works with whatever theme the old docs might've been using (e.g. mkdocs).

[mortenpi]

This is awesome!

1. I did notice that somehow the x is not visible with the old Documenter HTML theme:

   

   Probably some weird interaction with the CSS there. For the new theme and MkDocs it seems to work perfectly.

2. It skips the latest version? I guess the idea is that you'd fix that on master? But I think you might end up missing a fix for a version (e.g. 0.5.1 is the current latest, where it doesn't get applied, but the next tag is 0.6.0, which will be the first one that has it from Documenter). Could we just detect if the tag is there or not already?

3. Do you think it would be feasible to have a small test for this code as well, e.g. to update a simple sample HTML file and see if it matches a reference output? Just to test that the code actually runs.

[pfitzseb]

I can't repro the styling issue:

The other comments should be addressed, but Windows CI chokes on the symlink. Will figure out what's going on with that...

[mortenpi]

LGTM! The styling issue seems to be gone now too (even when building with the old commit), so I am not sure what was causing that.

Edit: Oh, I can repro the styling issue with the d00cf10 commit. So I think you already managed to fix the problem between me checking out this branch and posting the feedback 😛