First draft of the package to send quarto (and other serialized) emails

[juleswg23]

This PR sets up the development of the package to send emails authored by Quarto, in parallel with the writing of the email for data science guide. Mainly, tt adds core functionality of emailer-lib, as well as a test workflow and an API reference.

Emailer-lib supports the following types of functions:

• Email Ingress (Conversion to Intermediate Format)
• Email Egress (Sending)
• Email Structure Management - IntermediateEmail supports
  • HTML content
  • Plain text content
  • Subject lines
  • Multiple recipients
  • Inline attachments (images embedded in HTML)
  • External attachments (TODO)

CI & Testing

• Added a new test-emailer-lib job to .github/workflows/ci.yml, which runs tests for emailer-lib.
• Updated the main CI workflow to only trigger on pull requests and improved the docs build step to use quartodoc for API documentation generation.

Documentation & API Reference

• Expanded _quarto.yml to add a quartodoc section, generating a structured API reference.
• Added a README.md for emailer-lib, detailing its purpose, features, usage examples, and contribution guidelines.

Project Structure

• Added a top-level import structure in __init__.py, making all major functions and classes available directly from the package.
• Added a Makefile to emailer-lib for running tests and updating test snapshots easily.

[github-actions]

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