Broken links in README.md: "Fern" and "Fix"

### Problem Description

Two broken links have been identified in the "Example company blogs built with Headless" section of the `README.md` file.

The links that are currently not working are:

* **Fern**: `https://blog.buildwithfern.com` (This link appears under "Example company blogs built with Headless" as "Fern")
* **Fix**: `https://fix.security/blog` (This link also appears in the same section as "Fix")

Attempting to access these links results in an error, indicating that the pages are unavailable or cannot be found.

### Steps to Reproduce

1. Navigate to the `Hashnode/starter-kit` repository.
2. Open the `README.md` file.
3. Scroll down to the "Example company blogs built with Headless" section.
4. Click on the "Fern" link.
5. Click on the "Fix" link.

### Expected Behavior

Both links are expected to lead to their respective example blogs.

### Current Behavior

Both links result in an error page, preventing the viewing of the mentioned blog examples.

### Possible Solution

Review and update the URLs for the "Fern" and "Fix" links in `README.md` to point to the correct and functional resources.
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Thank you for all the amazing work you do with Hashnode. This starter kit is a valuable resource, and your platform empowers many developers to share their knowledge. I appreciate your continuous efforts!

Apri README.md e individua la sezione «Example company blogs built with Headless». Controlla gli URL di Fern e Fix, individua risorse attualmente funzionanti e aggiorna questi due link. L'attività è completata quando entrambi i link di README rimandano ai blog di esempio previsti.

Scritto dal modello di indicizzazione a partire dal testo della issue.