Nudge PRs to have linked issues, visual demos (#836)

* Nudge PRs to have linked issues, videos

* Accept suggestions

* Defer to material-ui docs and Notion

* typo

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,3 +1,7 @@
 <!-- Thanks so much for your PR, your contribution is appreciated! ❤️ -->
 
-- [ ] I have followed (at least) the [PR section of the contributing guide](https://github.com/mui/material-ui/blob/HEAD/CONTRIBUTING.md#sending-a-pull-request).
+- Closes <!-- Issue hyperlink -->
+
+- Visual demonstration <!-- Screenshot/Video link -->
+
+- Please follow the [the contributing guide on how to create great pull requests](https://github.com/mui/mui-toolpad/blob/master/CONTRIBUTING.md#sending-a-pull-request).

CONTRIBUTING.md

@@ -68,6 +68,10 @@ If you would like to hack on MUI Toolpad or want to run the latest version, you
   yarn prisma db push --accept-data-loss
   ```
 
+## Sending a pull request
+
+Please have a look at our general guidelines for sending pull requests [here](https://mui-org.notion.site/GitHub-PRs-7112d03a6c4346168090b29a970c0154) and [here](https://github.com/mui/material-ui/blob/master/CONTRIBUTING.md#sending-a-pull-request).
+
 ## Release process
 
 1. Generate a new version using: