feat: Add Custom Section Builder (Fixes #21)

[Vedhant26]

Hey @Mohit-368!

I' ve pushed a fix for Issue #21 to add the custom section builder. Taking your feedback from the previous PR into account, I made sure not to remove or override any of the existing standard sections!

Here's what I added:

• Users can now dynamically add multiple custom sections alongside the standard ones.
• Custom sections have their own title and Markdown content inputs.
• They fully support drag-and-drop reordering in the sidebar and have a dedicated delete button.
• They seamlessly integrate into the auto-generated Table of Contents and the final exported Markdown.

The core template structure remains completely untouched. Could you please take a look when you have a moment? If it looks good, I'd love to get this merged in!

Closes #21