…allow for easier maintenance, updated JSFiddle to use "latest" VFG, and created a CodePen version as well

• Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
  Docs Update

• What is the current behavior? (You can also link to an open issue here)
  Docs currently point to a JSFiddle that is no longer up-to-date (v2.2.1) and the core documentation is hosted on Legacy GitBooks.

• What is the new behavior (if this is a feature change)?
  JSFiddle has been updated and now points to the "latest" version of VFG (and should auto-update as it's no longer bound to a specific version).

Migrated the Legacy GitBooks documentation over to the vue-form-generator-docs repo, and synced this repo with a new GitBooks account that will auto-update from the GitHub repo. Documentation can now be updated through GitHub PR's and should hopefully stay more up to date.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)
  No

• Other information:

If you'd like to be added to the GitBook Team, send me a message with your email address. Apparently I can't just add GitHub users by username to the team. It requires me to send an "invite link".