docs: improve notarization and audit trail docs

[itsyaasir]

Description of change

This PR improves the public-facing documentation across the repository for both IOTA Notarization and IOTA Audit Trail.

It updates the audit-trail-rs and audit_trail_wasm documentation surfaces, reshapes the root README into an umbrella overview for both stacks, and adds concise Move package READMEs for audit-trail-move and notarization-move.

Links to any relevant issues

N/A

Type of change

• Bug fix (a non-breaking change which fixes an issue)
• Enhancement (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Fix

How the change has been tested

• cargo test -p audit_trail --doc
• cargo check --target wasm32-unknown-unknown

Change checklist

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have checked that new and existing unit tests pass locally with my changes