New documentation based on GitBook CLI

Split the root README.md in multiple Markdown files (in the `docs/` folder) to make easier reading, editing and extending the documentation. An online version is also available on netlify (https://qtpromise.netlify.com). Building it requires Node.js installed, then:

- npm install -g gitbook-cli
- gitbook install ./
- gitbook build . dist/docs

docs/assets/style.css

@@ -0,0 +1,15 @@
+a.anchorjs-link {
+    color: rgba(65, 131, 196, 0.1);
+    font-weight: 400;
+    text-decoration: none;
+    transition: color 100ms ease-out;
+    z-index: 999;
+}
+
+a.anchorjs-link:hover {
+    color: rgba(65, 131, 196, 1);
+}
+
+sup {
+    font-size: 0.75em !important;
+}