Bump mike from 1.1.2 to 2.1.3 in /docs

Bumps [mike](https://github.com/jimporter/mike) from 1.1.2 to 2.1.3.
- [Release notes](https://github.com/jimporter/mike/releases)
- [Changelog](https://github.com/jimporter/mike/blob/master/CHANGES.md)
- [Commits](jimporter/mike@v1.1.2...v2.1.3)

---
updated-dependencies:
- dependency-name: mike
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>