Update mkdocs requirement from ~=1.5 to ~=1.6 (#243)

Updates the requirements on [mkdocs](https://github.com/mkdocs/mkdocs) to permit the latest version.
- [Release notes](https://github.com/mkdocs/mkdocs/releases)
- [Commits](mkdocs/mkdocs@1.5.0...1.6.0)

---
updated-dependencies:
- dependency-name: mkdocs
  dependency-type: direct:production
...

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requirements_docs.txt

@@ -1,6 +1,6 @@
 invoke~=2.2
 mike~=2.1
-mkdocs~=1.5
+mkdocs~=1.6
 mkdocs-awesome-pages-plugin~=2.9
 mkdocs-material~=9.5
 mkdocstrings[python]~=0.25.1