CommonJS versus ESM docs

[zachleat]

Make sure to note all of the use cases for configuration files (CommonJS projects, ESM projects, CJS in an ESM project, ESM in a CJS project), template files (.11ty.js .11ty.cjs) etc etc.

Make sure to include notes on plugins too (you can import a CJS plugin!)

Probably reuse a lot of the release blog: https://www.11ty.dev/blog/canary-eleventy-v3/