Add logic to the workflow to build the docs zip

[a-maurice]

Description

Provide details of the change, and generalize the change in the PR title above.

Add logic to the linux workflow (since it is generally the fastest already) to generate the zip file containing the sources to document. Also updates the packaging logic to ignore it.

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Testing

Describe how you've tested these changes.

https://github.com/firebase/firebase-unity-sdk/actions/runs/2742344076

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Type of Change

Place an x the applicable box:

• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

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