Refactor /docs - Phase 2

[JasonAtClockwork]

I took a look at the styling too by running it locally. We'll want @randyclockwork to take a pass at some point, but I think this is fine for now.

I would like to take a second pass at this where we really try to distill down the basic concepts and put them together towards the top of the document. As it stands I think the table of contents is a little verbose before getting to the core concepts:

I would love to see the table of contents look more like:

Intro
    Getting Started
    Quickstarts
    Tutorials
Core Concepts
    Databases # I think we modify this to include modules as a section
    Tables
    Functions
    Subscriptions
    Authentication

Where if you expand all the sections it would look like:

Intro
    Getting Started
    Quickstarts
        React
        Vue
        HTML
        Unity
        Unreal
        Godot
        Rust
        ... etc
    Tutorials
        Chat App # With language selectors for all the languages
        Blackholio (Unity)
        Blackholio (Unreal)
Core Concepts
    Databases
        The Database Module
        Developing databases
        Building and publishing
        Transactions & Atomicity # discussion about atomic transactions and what they are (could also go in Functions)
        Automatic migrations # these could also go into an "Advanced usage section"
        Incremental migrations # these could also go into an "Advanced usage section"        
    Functions
        Overview
        Reducers
        Procedures
        Views
    Tables
        Defining tables
        Reading data
        Writing data
        Scheduled tables
        Performance best practices
        Advanced
            System tables
    Subscriptions
        Subscribing to tables
        Reducer callbacks # will become event tables
        Advanced semantics # should perhaps be embedded in one of the above
    Authentication
        SpacetimeAuth
        Clerk
        Auth0
        # ... etc
Other features
        PostgreSQL compatibility   

We might at some point need deeper levels of nesting and will need UI design for that.

Originally posted by @cloutiertyler in #3877 (review)