fix: replace 404 external link in data availability docs

[Copilot]

CoinMarketCap article URL returns 404. Replaced with working glossary link as specified in issue.

Changes

• Updated app/learn/celestia-101/data-availability/page.mdx line 20
• https://coinmarketcap.com/academy/article/what-is-data-availability → https://coinmarketcap.com/academy/glossary/data-availability-sampling-das

Original prompt

---

This section details on the original issue you should resolve

<issue_title>fix: link</issue_title>
<issue_description>```
--- Broken External Links ---
[404 - Not Found] https://coinmarketcap.com/academy/article/what-is-data-availability (in app/learn/celestia-101/data-availability/page.mdx)

replace the link that's broken and edit text to use this one https://coinmarketcap.com/academy/glossary/data-availability-sampling-das</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes fix: link #2400

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[jcstein]

@musalbas you wrote the original article this PR replaces (https://web.archive.org/web/20250823041924/https://coinmarketcap.com/academy/article/what-is-data-availability), LMK if you know where it is and we can swap it in. https://coinmarketcap.com/academy/article/what-is-data-availability