Update the crate description for identity-credential

[olivereanderson]

Description of change

Updates the crate description for identity-credential to contain a basic description, some simple examples and links out to other relevant pages such as our wiki and project level examples.

Links to any relevant issues

Issues #133 and #869 need this PR.

Type of change

Add an x to the boxes that are relevant to your changes.

• 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

Checked that the documentation looks OK with cargo doc --open (inside the identity-credentialdirectory) . All warnings reported are from other crates in the workspace.

Also runs locally under cargo test (which runs the examples in the README.md).

Change checklist

Add an x to the boxes that are relevant to your changes.

• 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
• New and existing unit tests pass locally with my changes

[olivereanderson]

All comments have been addressed.