Point API link to Ford docs directly

[SeanBryan51]

The api.md page (currently https://cable.readthedocs.io/en/latest/developer_guide/other_resources/api/) does not contain any useful information. This change points the API link under Developer Guide to the Ford documentation directly and removes api.md.

Type of change

• [x] New or updated documentation

Checklist

• [x] The new content is accessible and located in the appropriate section.
• [x] I have checked that links are valid and point to the intended content.
• [x] I have checked my code/text and corrected any misspellings

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📚 Documentation preview 📚: https://cable--442.org.readthedocs.build/en/442/

[SeanBryan51]

I have grepped the source code for api.md and didn't find any occurrences. The file is safe to remove.

[SeanBryan51]

@bschroeter I wasn't able to get a new window to open but I have now added API Reference to the main page