docs(expo): improve links and instructions for expo managed workflow

[gregfenton]

The links and descriptions of how to create a new Expo app and how to install RNF were incorrect.

These updates push more responsibility to the Expo docs themselves and reduce the amount that RNF needs to document.

Description

Related issues

Release Summary

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
  • Yes
• My change supports the following platforms;
  • Android
  • iOS
• My change includes tests;
  • e2e tests added or updated in packages/\*\*/e2e
  • jest tests added or updated in packages/\*\*/__tests__
• I have updated TypeScript types that are affected by my change.
• This is a breaking change;
  • Yes
  • No

Test Plan

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[mikehardy]

These look great to me - thank you!
CI seems to have something to say about markdown and spelling pretty frequently with docs changes but I can shepherd those through to merge pretty quickly while working on the repo today