New documentation setup (#1708)

* initial docsite setup

* switch from pushd to cd

* install and set up solidity-docgen

* use the docsite branch next for now

* make it clear that env var is a repository

* add a clarifying comment about a relative path

* change relative to absolute path in docsite script

* add docgen script

* add first few READMEs for contract documentation

* update solidity-docgen

* add docsite as dependency and adjust script

* update openzeppelin-docsite

* update solidity-docgen

* remove dummy text

* update docgen and docsite

* update openzeppelin-docsite

* add netlify.toml

* udpate tokens guide for 2.2

* add DOCUMENTATION.md

* Update docs/learn-about-utilities.md

Co-Authored-By: frangio <frangio.1@gmail.com>

* fix PaymentSplitter docs wording

* update solidity-docgen

* add missing ERC20 contracts

* update solidity-docgen

* trigger deploy with cleared cache

* update solidity-docgen

* update openzeppelin-docsite

* remove travis docs setup

* update openzeppelin-docsite

* switch to published solidity-docgen

docs/sidebar.json

@@ -0,0 +1,17 @@
+{
+  "Overview": [
+    "get-started"
+  ],
+  "Guides": [
+    "learn-about-access-control",
+    "learn-about-crowdsales",
+    "learn-about-tokens",
+    "learn-about-utilities"
+  ],
+  "API Reference": [
+    {
+      "type": "directory",
+      "directory": "api"
+    }
+  ]
+}