Update mike requirement from ~=2.0 to ~=2.1 (#244)

Updates the requirements on [mike](https://github.com/jimporter/mike) to permit the latest version.
- [Release notes](https://github.com/jimporter/mike/releases)
- [Changelog](https://github.com/jimporter/mike/blob/master/CHANGES.md)
- [Commits](jimporter/mike@v2.0.0...v2.1.1)

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

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements_docs.txt

@@ -1,5 +1,5 @@
 invoke~=2.2
-mike~=2.0
+mike~=2.1
 mkdocs~=1.5
 mkdocs-awesome-pages-plugin~=2.9
 mkdocs-material~=9.5