feat(pr template + readme): add link to contributor docs (#9884)

## **Description**

This PR aims to add a link to contributor docs in the PR template and in
the README, to foster adoption of contributor docs guidelines.

[Same PR for Extension
repo.](MetaMask/metamask-extension#25095)

## **Related issues**

Fixes: MetaMask/MetaMask-planning#1843

## **Manual testing steps**

1. Go to this page...
2.
3.

## **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 Coding
Standards](https://github.com/MetaMask/metamask-mobile/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
- [x] I’ve applied the right labels on the PR (see [labeling
guidelines](https://github.com/MetaMask/metamask-mobile/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.

.github/pull-request-template.md

@@ -35,7 +35,7 @@ Fixes:
 
 ## **Pre-merge author checklist**
 
-- [ ] I’ve followed [MetaMask Coding Standards](https://github.com/MetaMask/metamask-mobile/blob/main/.github/guidelines/CODING_GUIDELINES.md).
+- [ ] I’ve followed [MetaMask Contributor Docs](https://github.com/MetaMask/contributor-docs/blob/main/docs/pull-requests.md) and [MetaMask Mobile Coding Standards](https://github.com/MetaMask/metamask-mobile/blob/main/.github/guidelines/CODING_GUIDELINES.md).
 - [ ] 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

README.md

@@ -10,6 +10,8 @@ For up to the minute news, follow our [Twitter](https://twitter.com/metamask) or
 
 To learn how to develop MetaMask-compatible applications, visit our [Developer Docs](https://docs.metamask.io).
 
+To learn how to contribute to the MetaMask codebase, visit our [Contributor Docs](https://github.com/MetaMask/contributor-docs).
+
 ## Documentation
 
 - [Architecture](./docs/readme/architecture.md)