Rearrange all the docs to allow for a docs/major.minor/ setup

This PR rearranges almost all of the docs/ folder to better allow us to group actual documentation at getbootstrap.com/docs/4.0/. It builds on #22716 and would merge into that branch if accepted. The best way to take in the changes is to browse the branch.

Given the size of the diff, here's what's changed.

• The Jekyll source is no longer docs—it's the entire project.

• All the existing docs pages, minus the home page and about section are in docs/4.0/.

• The docs/4.0/ folder will be renamed (via git mv) with each minor/major release. Deploying these changes to our production site is a manual copy-pasta dance, so we'll never lose old versions of the core docs.

• We no longer have two dist directories. Previously, we had one in the root and one more duplicated via npm script in docs/ because Jekyll couldn't read from outside the source directory. This is the bulk of those 15,000 lines deleted.

• Moves some root dot files around. The .github is new and contains the contributing guidelines and issue template. The build directory contains the .htmllintrc and .htmlhintrc files.

• There are a ton more files and folders in our root directory. Not great, but not the end of the world to shave some code from the project.

I have no idea if this is the right approach, but it works for now. If this ends up shipping, we'll need to tackle a few more todos:

• Update all URLs within docs pages
• Update links to contributing docs and issue template. This will be done at launch.
• Look into consolidating more root directory files elsewhere (travis, hound, bable)
• Verify we need all these package manager files (sache.json, package.js, composer.json, etc)
• Consider nuking root changelog.md since we don't use it.
• Figure out if htmllint and htmlhint are both needed (real talk this messed with me a ton)

Gimme your thoughts—is there a better way?