Rest API: use externalDocs

[Maschga]

Fix #844

[Maschga]

In some descriptions, there are multiple Markdown urls.
See /state, for instance:

docs/static/rest-api.yaml

Lines 759 to 763 in 966aba1

	description: >
	Returns the complete state of the system. This structure is used by the
	UI. It can be filtered by [JQ](https://jqlang.github.io/jq/) to only
	return a subset of the data. [Read
	more.](https://docs.evcc.io/en/docs/reference/configuration/state)

However, externalDocs only allows one url to be provided:
OAI/OpenAPI-Specification#1034

Perhaps we should only make this change once the feature is available.

\cc @andig

[andig]

We could use it where already possible?