docs: run migration before starting Medusa server

[bytrangle]

As I understand, the Quickstart guide for setting a Medusa server uses SQLite, so it doesn't mention running migrations before running medusa develop.

This may trip up users who use PostgreSQL. Because the Quickstart page is the only page on creating a Medusa server, I added a small reminder about running migrations to seed Postgres after installing the server.

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[bytrangle]

I've changed the command to yarn run seed. This command, in addition to runing migrations if necessary, will seed the Postgres with dummy data. Without running it, when users run Gatsby storefront, it will fail because gatsby-node.js expects its GraphQL data to contain at least Medusa regions, products and collections.

[bytrangle]

@shahednasser Thank you so much for the feedback. Hope the new change is what you're looking for.