(chore) - Fix up react-urql examples (#808)
* Fix/document react-urql examples.
* Upgrade example dependencies to match latest URQL dependencies.
* Add README.md files where missing.
* Misc other tweaks to get the examples running.
Examples 1, 2 and 4 should now work first-time following the README instructions.
Example 3 still doesn't work in the browser. An error message shows: 'Error: Invalid hook call. Hooks can only be called inside of the body of a function component.'.
* Link to CodeSandbox from a react-urql example README.
* Build each package/exchange in parallel.