Restructure documentation

[frangio]

As part of a bigger review of the documentation (OpenZeppelin/docs.openzeppelin.com#47), we're restructuring the content. The idea is to organize it in a conceptual hierarchy rather than divide it in "Basics" and "Advanced".

The SDK docs will be split into CLI and Upgrades. The structure should be as follows. Most of these articles already exist, except the ones in italics.

CLI

• Overview (incl: Your first project)
• Compiling
• Create & Upgrade (incl: create2)
• Interacting with contracts
• Dependencies (i.e. “linking oz contracts”)
• Verify
• Admins (set-admin)
• Ethereum Packages (incl: “contracts architecture”?)
• Using with Truffle
• API Reference
  • Commands
  • Config Files

Upgrades

• Overview
• Programmatic library
• Writing upgradeable contracts
• Proxies (aka “Upgrades pattern”)
• Testing upgradeable projects ? (may want to remove this one)

The following should be moved to the "entry guides" section (which doesn't exist yet):

• Building a GSN powered dapp
• Deploying to a public network

The following should be moved to the forum:

• Upgrades governance
• Onboarding ERC20 tokens