Docs: Add model development to readme

[felickz]

Expanded support and documentation for data extensions:

• Updated issue creation instructions to cover both CodeQL queries and data extensions, including new template options, required fields, and guidance for modeling unmodeled libraries via YAML.
• Added a new entry in the issue template table for requesting CodeQL data extensions (models-as-data YAML), making it easier for users to find the correct workflow.

Improvements to review and workflow documentation:

• Clarified that pull requests may include data extensions (.model.yml files) in addition to .ql query files during review.
• Updated the description of issue templates to include both query and model requirements, reflecting the expanded scope of the template.

Language support table enhancements:

• Replaced the simple language list with a detailed table indicating which languages support query development and/or model development, highlighting expanded guidance for data extensions.

(also, enhanced repo for non unix shell compat)