docs: Provide sample code for handling Handle

[paulproteus]

This takes a recommendation from @kentonv 's recent conversation on sandstorm-dev and adds a link to @dwrensha 's GitLab code.

Review requested from at least one of:

• @dwrensha
• @kentonv

I'm willing to continue to make changes this, or use other people's preferred text, as needed.

[kentonv]

This recommendation only applies when packaging apps that are already coded around the assumption of a username that is a primary key. For new code I would recommend avoiding usernames as a primary key.

[paulproteus]

OK @kentonv how about now? :)

[kentonv]

I feel like this algorithm description is in the uncanny valley between precise and informal. How about:

If your app requires that handles be unique -- for example, because the app is designed to use "usernames" as the primary user identifier -- then you will need to track and store those handles in your app's storage. We suggest implementing the following algorithm, for each request:
1. If there is already a user account in your app's database associated with the request's `X-Sandstorm-User-Id`, use the handle associated with it.
2. Otherwise, if no existing user account is already associated with `X-Sandstorm-Preferred-Handle`, create a new account for this user's ID and handle, and use that.
3. Otherwise, this is a new user but their handle is already in use. Append a number to the handle such that it is unique, create a user account using the user's ID and the modified handle, and use that.