Add page for persistent alerts

[jwbonner]

Documentation for the new alerts API and widget.

[jasondaming]

I think the link is failing because if you want to use the "new" syntax I think you might need the full link. So either revert to the old style or change it to the full link and see if that works. See this section

[jwbonner]

I switched to the full link. Even with the old syntax I think it fails because the Python class doesn't exist yet.

Also failing on the full link because it 404s. Not sure how to handle that beyond just adding the robotpy bindings first...

[jasondaming]

Yeah makes sense I guess we just wait then till that happens. This looks ready to go otherwise to me.

[sciencewhiz]

I think I'd prefer to merge without a python link, and open an issue to have it added later when available.