Add validium example using avail js.

[0xSasaPrsic]

Pull Request type

Please add the labels corresponding to the type of changes your PR introduces:

• [ ] Feature
• [ ] Bugfix
• [ ] Refactor
• [ ] Format
• [x] Documentation
• [ ] Testing
• [ ] Other:

Description

Add an example of DA attestation using avail js.

Related Issues

Testing Performed

Checklist

• [ ] I have performed a self-review of my own code.
• [ ] The tests pass successfully with cargo test.
• [ ] The code was formatted with cargo fmt.
• [ ] The code compiles with no new warnings with cargo build --release and cargo build --release --features runtime-benchmarks.
• [ ] The code has no new warnings when using cargo clippy.
• [ ] If this change affects documented features or needs new documentation, I have created a PR with a documentation update.