[@kbn/config-schema] Add extends to the documentation

[maryam-saeidi]

🍒 Summary

This ticket is about adding missing documentation for extends to the config-schema documentation.

Since it can be used for intersections, it would be nice to add such an example as well: (Example provided by @jloleysens )

const a = schema.object({ a: schema.string() });
const bKeys = { b: schema.string() };
const b = schema.object(bKeys);

//  C = A & B (intersection of A and B)
const c = a.extends(bKeys);

And in general, it would be great to have a table similar to io-ts to show the type equivalent in Typescript.

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)