Update mkdocs requirement from >=1.0 to >=1.6.1 in /doc (#218)

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@1.0...1.6.1)

---
updated-dependencies:
- dependency-name: mkdocs
  dependency-version: 1.6.1
  dependency-type: direct:production
...

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Author: dependabot[bot]

doc/requirements.txt

@@ -1,4 +1,4 @@
-mkdocs>=1.0
+mkdocs>=1.6.1
 markdown-include
 pymdown-extensions
 pygments