Update Deploying to Heroku docs (gatsbyjs#14811)

The instructions for adding buildpacks here are going to result in errors and confusion.

`heroku buildpacks:set` sets a single buildpack so running it twice will override the first one.
I've changed the instructions to still run `heroku buildpacks:set heroku/nodejs` first which will ensure `heroku/nodejs` is the only buildpack set, then run `heroku buildpacks:add https://github.com/heroku/heroku-buildpack-static.git` so it will add it as a second buildpack. (the difference being `buildpacks:add` in the second command)

`app.json` is only honored for the heroku button and review apps so it should be specified as optional.
I replaced the ambiguous "sometimes this doesn't work" comment with more details about when you might want to use it and a link to the heroku platform api article which covers `app.json`.

docs/docs/deploying-to-heroku.md

@@ -4,7 +4,14 @@ title: Deploying to Heroku
 
 You can use the [heroku buildpack static](https://github.com/heroku/heroku-buildpack-static) to handle the static files of your site.
 
-Set the `heroku/node.js` and `heroku-buildpack-static` buildpacks on your application creating an `app.json` file on the root of your project.
+Set the `heroku/node.js` and `heroku-buildpack-static` buildpacks on your application.
+
+```shell
+$ heroku buildpacks:set heroku/nodejs
+$ heroku buildpacks:add https://github.com/heroku/heroku-buildpack-static.git
+```
+
+You can optionally add the buildpacks to `app.json` if you want to take advantage of the [heroku platform api](https://devcenter.heroku.com/articles/setting-up-apps-using-the-heroku-platform-api)
 
 ```json:title=app.json
 {
@@ -19,13 +26,6 @@ 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 with the following commands:
-
-```shell
-$ heroku buildpacks:set heroku/nodejs
-$ heroku buildpacks:set https://github.com/heroku/heroku-buildpack-static.git
-```
-
 Add a `heroku-postbuild` script in your `package.json`:
 
 ```json:title=package.json