docs: add useContent headings guide

[iitzIrFan]

What is it?

Solves: #6678

• Feature / enhancement
• Docs / tests / types / typos

Description

1. API Reference Update:
   A new section was added to the API reference file (index.mdx) to explain how useContent can retrieve headings from .mdx files. This included an example and practical use cases.

2. New Practical Guide:
   A new file, use-content-headings.mdx, was created in the main documentation section. This file provides a user-friendly guide on using useContent to retrieve page headings, complete with an example, notes, and use cases.

Checklist

• My code follows the developer guidelines of this project
• I performed a self-review of my own code
• I added a changeset with pnpm change
• I made corresponding changes to the Qwik docs
• I added new tests to cover the fix / functionality

[changeset-bot]

⚠️ No Changeset found

Latest commit: ac90eb9

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[AdrianRomanski]

Looks like duplicate #7303

[gioboa]

I think we need to change the position of this file.
.../docs/use-content-headings.mdx
all the other pages are inside a specific folder.

[iitzIrFan]

@gioboa Thanks for your review and yes had updates the position of the file.

[maiieul]

Hey @iitzIrFan thanks for the PR. Giorgio and I think it would make more sense to have it in /docs/guides/mdx/. Can you do that change please?

[iitzIrFan]

Sure @maiieul, updated the pr!

[maiieul]

Thanks @irfanullahjan but I meant we already have a route https://qwik.dev/docs/guides/mdx/. Please move your changes here and remove the changes from /api.

[maiieul]

The changes should be in the index.mdx

[iitzIrFan]

@maiieul PR updated! :)