Bump bl from 4.0.2 to 4.0.4 in /docs

Bumps [bl](https://github.com/rvagg/bl) from 4.0.2 to 4.0.4.
- [Release notes](https://github.com/rvagg/bl/releases)
- [Commits](rvagg/bl@v4.0.2...v4.0.4)

---
updated-dependencies:
- dependency-name: bl
  dependency-type: indirect
...

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

Author: dependabot[bot]