Move 'userland extensions' doc to a section w/in Examples

[jennifer-shehane]

I had difficulty finding the Typescript definitions the other day. I expected them to be in the examples area, but it's in this weird doc that's kind of out of place. Should be moved within examples.

[brian-mann]

This was fixed by #51 but you never explicitly said: "fixes #49" so Github never associated this issue to the PR.

If you add that language anywhere in a comment, the PR title, or the PR body it'll link and automatically close when the PR is merged.

[brian-mann]

Fixed by #51