Refactor headers and headings levels in customization.qmd

[chriscarrollsmith]

I think the document hierarchy got a bit jumbled here.

Here's a suggested heading hierarchy to improve document structure:

# Customization (h1)

## Development Workflow (h2)
### Dependency management with `uv` (h3)
### IDE configuration (h3)
### Testing (h3)
### Type checking with mypy (h3)
### Developing with LLMs (h3)

## Project Structure (h2)
### Customizable folders and files (h3)
### Quarto documentation (h3)

## Core Components (h2)
### Defining a web backend with FastAPI (h3)
#### Routing patterns in this template (h4)
### HTML templating with Jinja2 (h3)
#### Context variables (h4)
### Middleware exception handling (h3)
### Database configuration and access with SQLModel (h3)
#### Models and relationships (h4)
#### Database helpers (h4)
#### Cascade deletes (h4)

## Custom Theming with Bootstrap Sass (h2)
### Installation and setup (h3)
### Sass configuration (h3)
### Gulp compilation (h3)

## Form Validation Strategy (h2)
### Client-side validation (h3)
### Server-side validation (h3)

## Email Templating (h2)

## Writing Type Annotated Code (h2)
### Request models and validators (h3)
### Form data conversion (h3)

## Database Configuration and Access with SQLModel (h2)
### Models and relationships (h3)
### Database helpers (h3)
### Cascade deletes (h3)

Key changes from original:

1. Promoted "Core Components" to h2 to group architectural elements
2. Made "Custom Theming" its own h2 section
3. Demoted implementation details to h3/h4 under parent concepts
4. Separated form validation and email templating into their own h2 sections
5. Grouped type-related content under "Writing Type Annotated Code"
6. Gave database configuration a dedicated h2 section
7. Added clearer hierarchy for nested concepts like routing patterns and context variables