Release beta version of v2

[m-allanson]

• Create next.gatsbyjs.org and point it to v2
• Do a new v2 alpha release
• Do a new v1 release
• Merge master into v2
• Test v2 branch
• Branch v1 from master
• Add note about v2 beta to top of readme
• Add link to migration guide to top of readme
• Update v2--- netlify links to next.gatsbyjs.org
• Force pull master into v1
• Change netlify to build gatsbyjs.org from v1 branch
• add a robots.txt to the v2 branch (Add robots.txt to disallow indexing next.gatsbyjs.org #5952)
• Add a banner pointing from next.gatsbyjs.org to gatsbyjs.org ([v2][www] "You're viewing the docs for Gatsby v2 beta" banner #5915)
• Add docs on which branches to use for PRs while v2 is in beta ([v2] PR docs #5933)
• Resolve [v2] Fix v1 / v2 version conflicts #5899
• Do a fresh master -> v2 merge
• Publish final alpha release
• Merge v2 into master
• Update next.gatsbyjs.org to point to master
• Switch to lerna v3
• Publish beta 1 to npm
• Merge v2 beta blog post to v1 branch

[m-allanson]

Set up new.gatsbyjs.org as a generic "upcoming version of Gatsby" preview site? See new.babeljs.io.

[nihgwu]

what about next.gatsbyjs.org as we are using next tag for the coming v2

[KyleAMathews]

👍 to next since that's reusable. Also Babel use(d|s) new because it's a site rebuild and not specifically for the next version of Babel.

[KyleAMathews]

Beta is out!!! Thanks @m-allanson and @pieh and many others for all your contributions! Let's push on to a v2.