build(deps): [security] bump prismjs from 1.20.0 to 1.21.0 in /docs

Bumps [prismjs](https://github.com/PrismJS/prism) from 1.20.0 to 1.21.0. **This update includes a security fix.**
- [Release notes](https://github.com/PrismJS/prism/releases)
- [Changelog](https://github.com/PrismJS/prism/blob/master/CHANGELOG.md)
- [Commits](PrismJS/prism@v1.20.0...v1.21.0)

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