Document local extenders

[askvortsov1]

Followup to #194

@luceos did you have an article about this somewhere on Discuss, or am I misremembering?

[luceos]

I have some information scattered and a repo with just one example.

[franzliedke]

We started with local extenders before we dive into full extensions: https://docs.flarum.org/extend/start.html#hello-world

[askvortsov1]

We started with local extenders before we dive into full extensions: https://docs.flarum.org/extend/start.html#hello-world

Nice catch! Added a link and a blurb to the placeholder page in the PR.