Update field colour readme to include the information currently on devsite

[rachel-fenichel]

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Component

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Problem

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Request

Field-colour will only exist as a plugin as of v11, so its documentation is being moved off devsite and into its README.

There's currently a README file, but the docs on devsite are more comprehensive. This bug tracks combining the two into a single coherent form.

Alternatives considered

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Additional context

https://github.com/google/blockly/issues/7878 points out an issue in the existing devsite documentation.