fix(docs): update Heroku deployment instructions (gatsbyjs#11601)

## Description

- Added CLI instructions for adding Heroku buildpacks
- Removed misleading instructions.

docs/docs/deploying-to-heroku.md

@@ -19,7 +19,12 @@ Set the `heroku/node.js` and `heroku-buildpack-static` buildpacks on your applic
 }
 ```
 
-Sometimes specifying buildpacks via the `app.json` file doesn’t work. If this is your case try to add them in the Heroku dashboard or via the CLI.
+Sometimes specifying buildpacks via the `app.json` file doesn’t work. If this is your case, try to add them in the Heroku dashboard or via the CLI with the following commands:
+
+```sh
+$ heroku buildpacks:set heroku/node
+$ heroku buildpacks:set https://github.com/heroku/heroku-buildpack-static.git
+```
 
 Add a `heroku-postbuild` script in your `package.json`:
 
@@ -31,24 +36,6 @@ Add a `heroku-postbuild` script in your `package.json`:
 }
 ```
 
-It is worth noting that, by default, creating a new project with `gatsby new <your-project-name>` creates a `package.json` with a `start` script as `npm run develop`. Without creating a Procfile to specify your commands to run for your process types, Heroku will run the web process via the `npm start` command by default. Since we've compiled our production-optimized build with `gatsby build`, the command we want to run is actually `gatsby serve`. To do this, either add the following script to your `package.json`:
-
-```json:package.json
-{
-  "scripts": {
-    "start": "gatsby serve --port ${PORT}"
-  }
-}
-```
-
-Or, create a `Procfile`:
-
-```title=Procfile
-web: gatsby serve --port ${PORT}
-```
-
-The port needs to be specified to use the Heroku dyno's runtime environment variable in order to bind to the port correctly.
-
 Finally, add a `static.json` file in the root of your project to define the directory where your static assets will be. You can check all the options for this file in the [heroku-buildpack-static](https://github.com/heroku/heroku-buildpack-static#configuration) configuration.
 
 ```json:title=static.json