Add dropdown/collapsible components for page content

[bveeramani]

We need dropdown/collapsible components that can be used within documentation pages to hide and show content.

What to build:
A collapsible section component that users can add to their markdown documentation. Think of it like an accordion or details/summary element - you see a title, click it, and more content expands below.

Where to work:

• Create a new component in src/luma/app/components/ (e.g., Dropdown.tsx or Collapsible.tsx)
• Add Markdoc configuration in src/luma/app/markdoc/ so it can be used in markdown

What it should do:

• Show a title/heading that's always visible
• Click the title to expand/collapse the content
• Show a visual indicator (chevron/arrow icon) that rotates when open/closed
• Smooth animation when opening/closing
• Support any markdown content inside (text, code blocks, lists, etc.)

Example usage in markdown:
Users should be able to write something like:

{% dropdown title="Advanced Configuration" %}
Here's some advanced content that's hidden by default...
{% /dropdown %}

Technical approach:

1. Create the React component with open/closed state
2. Register it as a custom Markdoc tag (check src/luma/app/markdoc/ for examples)
3. Style it with Tailwind CSS

Reference:
Look at how CodeBlock.tsx is implemented and registered in the Markdoc config - you'll follow a similar pattern!