build(deps): bump markdown from 3.7 to 3.8 in /docs (#3312)

Bumps [markdown](https://github.com/Python-Markdown/markdown) from 3.7 to 3.8.
- [Release notes](https://github.com/Python-Markdown/markdown/releases)
- [Changelog](https://github.com/Python-Markdown/markdown/blob/master/docs/changelog.md)
- [Commits](Python-Markdown/markdown@3.7...3.8)

---
updated-dependencies:
- dependency-name: markdown
  dependency-version: '3.8'
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]