docs: Add a guide on how to define the configuration schema for a new connector

[edgarrmondragon]

Documentation type

How-to guides

Description

It should

• mention that a default=... means the config dict will always have a value for the setting, so it should be retrieved with square brackets: self.config["<setting name>"]
• encourage developers to mark settings as required=True when the connector can't be executed without them. These settings should also be retrieved with self.config["<setting name>"].

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