docs: add guide on how to use the existing yaml and meta file to have a code-first approach of using hasura

Let's make it more fun using Hasura!

The code first approach way of using Hasura means:

> Managing everything related to Hasura (DB and Hasura features like (permission and relation)) via YAML and META file

Benefits including:
1. All benefits of adopting Infrastructure as Code
1. Can be version controlled
1. Easy replicating
1. The big picture in a glance
1. Enable future VSCode / WebStorm plugging for linting for better DX
1. Better CI/CD integration

In short, scalable and maintainable.

The current setup of Hasura already supports it. YAML file for DB, and META file for permission and relation.

But what about a section purely for using Hasura in a code-first way with an example:

Let's extend the `current Slack app permission example` on how to use it in a code-first way.

The tutorial might contain the following flow basically covers a lifecycle of an application:

1. not touch GUI
1. create DB schemas
1. create Hasura relations
1. create Hasura permissions
1. initialize the DB with the created yml and meta
1. Change schema / permission / relationship
1. Apply migration
1. Different `schema migration` handling:
1. new field
1. update the existin:g field with data
1. `data migration`: how to migrate the existing data to the new schema
1. Show some examples for handling/migrating different kind of request type on this [page](https://hasura.io/docs/1.0/graphql/manual/api-reference/schema-metadata-api/index.html#query)
1. Doing a rollback
1. Apply `dev` DB changes to `uat` / `production` DB

This is currently what I am thinking of. Thanks