Add redirect to is-land plugin from partial-hydration

[rdela]

Both 11ty.dev and zachleat.com link to: https://www.11ty.dev/docs/plugins/partial-hydration/

in several places. Which is now: https://www.11ty.dev/docs/plugins/is-land/

This seems to be the easiest fix, unless I am missing something.

[rdela]

Renamed here https://github.com/11ty/11ty-website/commit/b844a7f7db13c4c23f762932942d72e12a46ebb2

[zachleat]

Thank you!