Refactor and simplify styles and templates

[rogermparent]

This PR has three main goals:

• Remove non-Tailwind PostCSS plugins and have Tailwind fill their place
• Refactor our layouts into more standard Gatsby layouts
• Allow HTML defaults to take care of features they do best, stripping away JS that gets in the way

This could have been broken up into more distinct steps, but ultimately seemed like it would be more work to put in the effort to get one part working only to immediately follow up with removing the part we were adapting to- thus, this monster of a PR.

This PR also makes the homepage look a bit better.

One integrated into the respective repos, this PR's changes should fix quite a few issues:

• https://github.com/iterative/iterative.ai/issues/259
• Browser back should be taking to the same place on the page #64

[rogermparent]

Thanks again for the review!

Scrollbars are fixed, just needed a CSS tweak on code blocks that must have gotten clobbered.

Fixed up the args linker, it was as simple as pointing it to /doc instead of /docs. The defaults should just work with all our sites, but I also made that option configurable in case another site has different requirements.

[yathomasi]

Great work so far. I think we are almost ready to merge this. There seems to be a small padding issue on the mobile view:

1. https://gatsby-theme-replace-po-cudk5d.herokuapp.com/doc/start
   (seems to have larger padding shrinking the content)
2. https://dvc.org/doc/start

I think, we can either have the padding removed on mobile view or use max-width method similar to dvc.org right now.

[rogermparent]

I just pushed up a fix to the padding, I had replaced a media query that removed horizontal padding on the content in mobile view without making the tailwind rules that replace it do the same, and the change was subtle enough that I didn't pick it up looking back at it.

Thanks again for the great reviews @yathomasi! Sorry there's been so much back-and-forth, I definitely got lost in this PR after a while. I fixed the padding by simply removing px on content on screens below xs, which is currently our mobile breakpoint.

Taking a tangent on xs as a mobile breakpoint: the website is a bit squished looking right at the breakpoing, maybe we could consider using sm in the future and dropping xs considering it's not even a default tailwind screen size.

[yathomasi]

With the recent change to allow docsPrefix, the value provided through slug props had changed. It caused the dysfunction of the next and prev buttons and also the Page Title. I have updated to use pathname instead of slug.

[yathomasi]

I checked thoroughly and everything looks good to me. Finally, we should go ahead and merge this.

Also, I think we should bump a minor version with all these changes.

[yathomasi]

I think I mentioned it earlier too that it would be easier/better if we could use the default values for screen sizes that tailwind uses. But, it's not a blocker as we can continue using the existing tailwind config on consumer websites.

[rogermparent]

Yup! we can follow up after. I decided to port the defaults early on in the process in case anything precise would be broken otherwise.

[rogermparent]

Bumping to v0.2.0 and merging! Thanks for doing the quick edit to speed this along.