[docs] Add __typename example to advanced entity docs

[rhino88]

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In some cases it may be useful to define an entity as a singleton. The @key directive supports the __typename meta field to declare an entity as a singleton.

In the following example, the Meta entity can be uniquely identified by its __typename (because there is only ever one instance):

type Meta @key(fields: "__typename") {
    version: String!
}

I was able to confirm this works as expected in a repro: https://codesandbox.io/p/sandbox/tender-engelbart-wympgk?file=%2Ftsconfig.json%3A1%2C1

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[Meschreiber]

Thanks for this @rhino88 ! The team is discussing whether this is a pattern we want to promote, we'll keep you updated.

[rhino88]

Thanks @Meschreiber.

This is something we've found useful for a number of scenarios in production (e.g HomePage entities). The only current alternative we found would be to introduce another dummy field to use as a @key, which isn't really desirable.