docs(installation): add missing pgrx initialization and database connection steps

[SheteUC]

What kind of change does this PR introduce?

doc update

What is the current behavior?

Installation Documentation missing some steps "cargo pgrx init ... start ... connect" #403

What is the new behavior?

This update includes the necessary steps to initialize pgrx, start the PostgreSQL database, and connect to it before enabling the pg_graphql extension. These steps address the gap in the installation process for new users, ensuring a smoother setup experience.

Additional context

Closes #403

[imor]

@SheteUC thanks for the PR. If you could fix the CI failure and tackle my review comments, I'd be happy to merge.