Bump image-size from 1.1.1 to 1.2.1 in /docs

Bumps [image-size](https://github.com/image-size/image-size) from 1.1.1 to 1.2.1.
- [Release notes](https://github.com/image-size/image-size/releases)
- [Commits](image-size/image-size@v1.1.1...v1.2.1)

---
updated-dependencies:
- dependency-name: image-size
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]