Make documentation for forecasts signal agnostic

[unitrium]

Pull Request

Issue Number: Link to Github Issue or Azure Dev Ops Task/Story)

Summary

Make the documentation for forecasts, signal agnostic.

Changes

• In the carbon aware sdk documentation, remove the mention of marginal as if you use ElectricityMaps' signals, the signals you will get are not marginal forecasts but average carbon intensity. To avoid any confusion, making the documentation signal agnostic.
• Update the carbon aware API documentation accordingly.

Checklist

• Local Tests Passing?
• CICD and Pipeline Tests Passing?
• Added any new Tests?
• Documentation Updates Made?
• Are there any API Changes? If yes, please describe below.
• This is not a breaking change. If it is, please describe it below.

Are there API Changes?

If yes, what are the expected API Changes? Please link to an API-Comparison
workflow with the API Diff.

Is this a breaking change?

No, only documentation changes.

Anything else?

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to link Pull Requests to Issues via keywords

This PR Closes #612

[savgoodman]

+1 for this pull request - I believe we've aligned through SCI discussions that we should not be exclusively highlighting marginal emissions metrics given the challenges and limitations of that data.

[danuw]

LGTM - Thank you