Plugin authoring documentation

[zachleat]

We have https://www.11ty.dev/docs/plugins/ which surfaces plugins but there is no section about authoring.

• Also reference the 11ty compatibility versionCheck trick to replace peerDependencies: https://github.com/11ty/eleventy-dev-server/issues/5#issuecomment-1063409355 from https://github.com/11ty/eleventy/issues/2001

Related: #405 #1681

[zachleat]

Cross referencing this excellent PR from @j-f1 https://github.com/11ty/11ty-website/pull/1338