improve resources references in README

[fmigneault]

Description

• Add pystac.extensions.mlm reference (relates to Add MLM extension stac-utils/pystac#1542)
• Update listing of resources with more details and formatting

Note

Reference to "pystac v1.13.0" is defined. This is not yet released, but expected to be the next version.
Waiting on this to be released to validate and merge...

Related Issue

• Add MLM extension stac-utils/pystac#1542

Type of Change

• 📚 Examples, docs, tutorials or dependencies update;
• 🔧 Bug fix (non-breaking change which fixes an issue);
• 🥂 Improvement (non-breaking change which improves an existing feature);
• 🚀 New feature (non-breaking change which adds functionality);
• 💥 Breaking change (fix or feature that would cause existing functionality to change);
• 🔐 Security fix.

Checklist

• I've read the CONTRIBUTING.md guide;
• I've updated the code style using make check;
• I've written tests for all new methods and classes that I created;
• I've written the docstring in Google format for all the methods and classes that I used.