Fix: Decrase anchor validation treeprocessor prioriy

[joapuiib]

This PR is a copy of the work on mkdocs/#3695

Resolves mkdocs/#3690

(...) the suggested solution is to decrease the treeprocessor processor priority to 3.9:

https://github.com/mkdocs/mkdocs/blob/fb1d106747930b094bc464189d4b1c349fc541c6/mkdocs/structure/pages.py#L343

[oprypin]

Thanks.

I'll get to fixes significantly later - after the renaming and such stuff is fully done.

Regarding this fix, I just remember what I was looking at previously from a very long time ago. There was a reason that the priorities are in that exact order. Moving the priorities like this fixes the conflict in this place but creates a conflict in another place, I'm pretty sure. So I'll look at it later to check this.