Add API doc for supported series

[wallyworld]

We introduce a new supported-series endpoint.

[jujugui]

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[rogpeppe]

s/entity/charm/ ?

I think I'd call this endpoint charm-supported-series, as it's charm-specific,
and like other charm-specific endpoints, it'll return a metadata-not-found error when invoked on bundles.

Also, we should be clear about the behaviour when the charm metadata does not support multiple series.

[wallyworld]

I've renamed to charm-supported-series and added an extra example to show how an empty list is returned if no supported series are declared in the metadata.

[jujugui]

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[jujugui]

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