move all models into subdirectories of ./models

- this required an update to the invoke-ai fork of gfpgan
- simultaneously reverted consolidation of environment and
  requirements files, as their presence in a directory
  triggered setup.py to try to install a sub-package.

requirements-mkdocs.txt

@@ -0,0 +1,3 @@
+mkdocs
+mkdocs-material>=8, <9
+mkdocs-git-revision-date-localized-plugin