Add new docs theme, and restructure (#978)

* Change documentation theme to 'furo'
* Update docs source for new theme
* Update index and split introduction
* Add a Getting Started section to the docs
* Rename getting started docs file
* Reorganise setup instructions
* Update usage sections for consistency
* Update docs references in main README
* Show release in sidebar

docs/source/_static/css/custom.css

@@ -1,27 +0,0 @@
-/*
- * Custom CSS Rules for Sphinx RTD Theme
- */
-
-.kbd {
-    background-color: #eeeeee;
-    border: 1px solid #b4b4b4;
-    border-radius: 3px;
-    color: #333333;
-    display: inline-block;
-    font-size: 0.85em;
-    font-weight: 400;
-    line-height: 1;
-    padding: 2px 4px;
-    margin-left: 1px;
-    margin-right: 1px;
-    white-space: nowrap;
-}
-
-.tight-table td {
-    white-space: normal !important;
-    vertical-align: text-top;
-}
-
-pre span.go {
-    color: #333333 !important;
-}