New year, new docs!

[tomreitz]

This PR is an overhaul of the earthmover docs. Specific changes include:

• Switch from a single README.md file to a docs site driven by Material & MkDocs (the same framework used for enabledataunion.org), which can be deployed to GitHub Pages via an automation, and which comes with a lot of nice features (like docs search). All the content from the old README.md has been moved into the docs site (with some slight edits and formatting changes). The docs site can be served locally with pip install mkdocs-material && mkdocs serve.

• Switch to new EA branding/logo for earthmover (1).
• Split the content from README.md (which was getting quite long) into several pages (2).
• Organize headings on various pages to make in-page Table of Contents (left side) cleaner and facilitate docs section linking.
• Add a gif demo and quick-start guide to "Home" page.
• Add a "Learn" page to help new users become more familiar with the building blocks of earthmover, YAML and Jinja.

• Change verbiage to better align to dbt: "earthmover project" and "packages" (instead of "bundles")
• Add configuration examples for various different types of sources on the "Project configuration" page.

• Add "Comparison to dbt" section to "Design" page.
• Use Material features like Admonitions and Content tabs throughout.

My hope is that these improvements to the earthmover documentation will both be helpful to users and make it easier to continue to expand the docs as earthmover development continues.

If this PR is merged to main, I believe the docs should automatically appear at https://edanalytics.github.io/earthmover. README.md is now a thin description of earthmover pointing to that link, so when deployed, other places that show README.md (like PyPI) should also direct folks to the new docs site.

I would recommend that reviewers check out this branch, stand up these docs locally, and review them in detail. Feel free to leave comments about specific content changes, as well the general approach. Thanks!