Write out Tandems goals/roadmap in the manifesto docs

[pirog]

What is to be done?

Right now we have two contentless sections for "Goals" and "Roadmap" in the manifesto docs
https://tandem-docs.lndo.site/manifesto/

We probably should consolidate them into a single section and list high level goals and roadmap things on a regular (quarterly, yearly? basis)

This is a basically a placeholder task for @pirog and @reynoldsalec

Task checklist

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