Add a specification for invitation URLs

https://github.com/earthstar-project/earthstar/issues/36 describes an invitation URL format which has now received a second iteration. This invitation URL format makes it possible to produce single codes which can be exchanged between users and parsed by Earthstar applications, and there is an open PR for adding utilities for these codes to Earthstar's main API (https://github.com/earthstar-project/earthstar/pull/313).

There should be a specification for creating and parsing these codes on the website!

Start by reading issue #36 for the invitation URL format and PR #313 for the related Earthstar API utilities. Document how these codes are created and parsed on the website, incorporating the second iteration; it is done when the specification covers the format clearly and matches the API work.

Written by the indexing model from the issue text.