The next version will be v3.6.

• Copy v3.5 folder over as a base
• Set Draft warning (so readers will know it is cutting edge documentation)
• Set the etcd.io/docs/next redirect rule to point to the new v3.6 folder
• Update the symlinks in the Makefile (for the link checker)
• Consider updating the README to reflect what you've done :) -- see README: section "Releasing a new version of the etcd documentation" is out-of-date #406

Related: #252