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: Peer dependency error with plugin eleventy-dev-server#5 (comment) from Plugin compatibility with prereleases #2001

Related: #405 #1681

[zachleat]

Cross referencing this excellent PR from @j-f1 11ty/11ty-website#1338