Add "Function prototype methods" section to Actions guide

[julesyoungberg]

Description (required)

Adds a new "Function prototype methods" section to the Actions guide.

This describes the restrictions introduced by withastro/astro#14248.

Related issues & labels (optional)

• Closes #
• Suggested label: new content

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Warnings reference

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🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

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