Getting to v4

[mdo]

Long story short, v4 has taken far too long and I know we're struggling to find enough time to make significant dents across the open issues and PRs. To help get through it all, I'm calling it on v3 and outlining the big steps remaining here so folks know what's in store.

Here's how we get from a string of alphas to a finalized v4 release:

• Stop all work on v3—today. The open issues, split dev setup, and more holds us back from focusing entirely on v4. I'll close all remaining v3 issues and milestones. Any new changes to v3 will be sporadic and highly irregular.
• Switch v4-dev to the default branch. Building on step one, this makes v4-dev the code you see when you visit the repo. I'll cut a v3-dev branch and leave master as-is until v4 ships.
• Move the docs sites. The biggest pain will be moving our docs pages around (specifically, moving v4-alpha.getbootstrap.com to getbootstrap.com). The current v3 docs will be moved to a sub-directory like our other hosted versions.
• Leave the Alpha site as-is, and introduce new docs subdirectory. We'll leave v4-alpha.getbootstrap.com as a snippet of history (updating the header of course to point to the stable release) and put the new docs at getbootstrap.com/docs/4.0/.
• Ship some betas. After the alpha releases take care of the major breaking changes, we'll ship some beta releases to work out the final kinks before RC and final releases.

There's a ton this list misses, but these are the broad strokes that get us to v4. Holler with any questions or comments as I push us through this.

/cc @twbs/team