Feature: Organize README.md file using drop-downs

The README is getting quite long and hard to navigate. We should organize it
using collapsible ``/`` dropdown sections.

## Motivation
The README has grown significantly — scrolling to find relevant sections is cumbersome.

## Proposed Solution
Use native HTML `` and `` tags (supported by GitHub Markdown)
to collapse sections like setup instructions, environment variables, feature flags, etc.

## Example

Environment Setup

...content...