Docs: Update fastify.md with Global outputs

[rumoursbehindme]

Description

Checklist:

• I have read the CONTRIBUTING document and have signed (or will sign) the CLA.
• I have updated/added documentation affected by my changes.
• I have added tests to cover my changes.

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Latest commit: 485e13f

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• ✅ login: rumoursbehindme / name: Rajeev Hegde (91bf5d7, 485e13f, c30cb1f, 1a49461)

[LuLaValva]

Thanks for the docs addition! For the future we are recommending that devs use $global instead of out.global, but otherwise looks great!

Also, it may be worth linking to this page in case people are confused about serialization.

[LuLaValva]

Looks great, thanks @rumoursbehindme!

[DylanPiercey]

Thanks @rumoursbehindme