docs(deploying-to-firebase.md): add firebase.json cache configuration (#21897)
docs(deploying-to-firebase.md): add firebase.json cache configuration (#21897)

* docs(deploying-to-firebase.md): add firebase.json cache configuration

Adds how to update the `firebase.json` to configure the firebase cache correctly.

* chore: format

Co-authored-by: GatsbyJS Bot <313a651c59fdaade90d152972001707d94b0975c@gmail.com>

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