Document how to protect endpoints

[stevekrouse]

Given that HTTP vals are always open to the public, users will want ways to secure their endpoints.

We explain super briefly how to do protect endpoints here: https://docs.val.town/reference/permissions/

But we can do better and add more methods.

They're a bit janky, but I love using:

• https://www.val.town/v/stevekrouse/lastlogin
• https://www.val.town/x/std/oauth

There might be lightweight ways to make them less janky, ie transfering or remixing lastlogin into std, but regardless we should document them because they are useful. They're somewhat the equivalent of Cloudflare's Zero Trust.