add ML-Model migration guide

[fmigneault]

Description

• Add ML-Model Legacy document providing migration guidance from the deprecated ML-Model extension.
• Move DLM Legacy document together under a common doc/legacy directory.

Related Issue

• relates to deprecated in favor of MLM extension ml-model#16

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.

[m-mohr]

I have no clue about this, but I appreciate the effort and I'm happy that we can provide this for the users.