Update Deploying to Heroku docs (#14811)
Update Deploying to Heroku docs (#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`.
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