Docs: Update Developer Docs link in README (#29733)

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## **Description**

This link was pointing to a dead site, this fixes it to point to the
correct URL.

[![Open in GitHub
Codespaces](https://github.com/codespaces/badge.svg)](https://codespaces.new/MetaMask/metamask-extension/pull/29733?quickstart=1)

## **Related issues**

N/A

## **Manual testing steps**

1. Go to
https://github.com/MetaMask/metamask-extension/blob/develop/docs/README.md
2. Click the link to Developer Docs 

## **Screenshots/Recordings**

<!-- If applicable, add screenshots and/or recordings to visualize the
before and after of your change. -->

### **Before**

<!-- [screenshots/recordings] -->

### **After**

<!-- [screenshots/recordings] -->

## **Pre-merge author checklist**

- [x] I've followed [MetaMask Contributor
Docs](https://github.com/MetaMask/contributor-docs) and [MetaMask
Extension Coding
Standards](https://github.com/MetaMask/metamask-extension/blob/main/.github/guidelines/CODING_GUIDELINES.md).
- [x] I've completed the PR template to the best of my ability
- [ ] I’ve included tests if applicable
- [ ] I’ve documented my code using [JSDoc](https://jsdoc.app/) format
if applicable
- [ ] I’ve applied the right labels on the PR (see [labeling
guidelines](https://github.com/MetaMask/metamask-extension/blob/main/.github/guidelines/LABELING_GUIDELINES.md)).
Not required for external contributors.

## **Pre-merge reviewer checklist**

- [ ] I've manually tested the PR (e.g. pull and build branch, run the
app, test code being changed).
- [ ] I confirm that this PR addresses all acceptance criteria described
in the ticket it closes and includes the necessary testing evidence such
as recordings and or screenshots.

docs/README.md

@@ -8,7 +8,7 @@ For help using MetaMask, visit our [User Support Site](https://support.metamask.
 
 For up to the minute news, follow our [Twitter](https://twitter.com/metamask_io) or [Medium](https://medium.com/metamask) pages.
 
-To learn how to develop MetaMask-compatible applications, visit our [Developer Docs](https://metamask.github.io/metamask-docs/).
+To learn how to develop MetaMask-compatible applications, visit our [Developer Docs](https://docs.metamask.io/).
 
 - [How to add custom build to Chrome](./add-to-chrome.md)
 - [How to add custom build to Firefox](./add-to-firefox.md)