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

scripts/docgen.sh

@@ -0,0 +1,6 @@
+#!/usr/bin/env bash
+
+OUTDIR=docs/api
+
+rm -rf "$OUTDIR"
+solidity-docgen -o "$OUTDIR" -i contracts/mocks -i contracts/examples