docs: BLS Aggregator Service

[damiramirez]

What Changed?

This PR adds documentation to the lib.rs file. We use simple_mermaid to render a Mermaid diagram in docs.rs. Also create a new target make docs to generate the workspace documentation.

To review the docs of BLSAggregator:

cargo doc -p eigen-services-blsaggregation --no-deps --open

Related #380

Reviewer Checklist

• New features are tested and documented
• PR updates the changelog with a description of changes
• PR has one of the changelog-X labels (if applies)
• Code deprecates any old functionality before removing it