Replace ember-cli-tailwind with postcss and tailwindcss

[josemarluedke]

Fixes #407.

cc/ @samselikoff @rwwagner90

@samselikoff Thanks for the video on EmberMap on this migration, that really helped.

[RobbieTheWagner]

Thanks so much for the PR @josemarluedke! @samselikoff let me know if everything looks okay to you, and we can merge

[samselikoff]

Awesome work @josemarluedke! Thanks for doing all this work!

Future PR idea: wonder if there's a way for us to distribute just the dist of this tailwind build into the final host apps. No reason for docs apps to have to pay the build-time cost of compiling this every time. Would be amazing to just ship the built (+ even purged!) tailwind docs styles.

[alexlafroscia]

wonder if there's a way for us to distribute just the dist of this tailwind build into the final host apps

This would also solve the issue of #406, where the addon's instance of Tailwind and the ember-cli-addon-docs instance are conflicting.

[nadnoslen]

Thank you for this!

[ef4]

It would be good to get this released. I think without it addon-docs is generating invalid CSS for anyone on ember-cli >= 3.13.

[NullVoxPopuli]

I think this PR broke default styles.
See: #453