Migrate documentation to github pages (#1523)

Switch from having all documentation contained within the README to
publishing a jekyll static site of documentation under github pages.

This allows for configuration and installation guides to be handled
separately to make for the relevant pieces to be in smaller chunks and
therefore easier to follow for users. Additionally a table of contents
can now be included in a left navigation section that ensures it should
be possible to quickly jump from the start to any section and back
again.

Include support for publishing previews and releases under separate
directories to allow for them to exist at the same time as the other
latest version of the documents.

The navigation section also includes support for accessing any of the
other versions published so that it easier to see what configuration
options exist for a given release. These will be published automatically
when releases are added.

docs/_sass/custom/custom.scss

@@ -0,0 +1,84 @@
+
+$callouts: (
+    info: ($blue-000, rgba($blue-200, .2), 'INFO'),
+    warn: ($yellow-300, rgba($yellow-200, .2), 'WARNING'),
+    danger: ($red-300, rgba($red-000, .2), 'DANGER')
+);
+
+@each $class, $props in $callouts {
+    .#{$class} {
+        background: nth($props, 2);
+        border-left: $border-radius solid nth($props, 1);
+        border-radius: $border-radius;
+        box-shadow: 0 1px 2px rgba(0, 0, 0, 0.12), 0 3px 10px rgba(0, 0, 0, 0.08);
+        padding: .8rem;
+
+        &::before {
+            color: nth($props, 1);
+            content: nth($props, 3);
+            display: block;
+            font-weight: bold;
+            font-size: .75em;
+            padding-bottom: .125rem;
+        }
+    }
+}
+
+// override site-title padding to ensure logo is better sized
+.site-title {
+    padding-top: 0.125rem;
+    padding-bottom: 0.125rem;
+}
+@media (min-width: 50rem) { .site-title { padding-right: 1rem; padding-left: 1rem; } }
+
+// override search style for dark to ensure default text is readable
+@media (prefers-color-scheme: dark) {
+    input#search-input {
+        color: white;
+        &:focus {
+            color: white;
+        }
+    }
+    #search-input::placeholder {
+        color: $body-text-color;
+        opacity: 0.6;
+    }
+}
+
+// tweak the toc plugin styles to keep toc in nav more left aligned
+ul#toc ul {
+  padding-left: 0.6rem;
+}
+
+ul#toc {
+  padding-left: 2.75rem;
+}
+
+ul#toc li {
+  margin-bottom: 0px;
+  margin-top: 0px;
+}
+
+.toc-entry a {
+/*
+  background: initial;
+  padding-top: 0.1rem;
+  padding-bottom: 0.1rem;
+  display: block;
+  padding-right: $gutter-spacing-sm;
+  line-height: 1.5rem;
+  text-decoration: none;
+  &.active {
+    font-weight: 600;
+    text-decoration: none;
+  }
+  &:hover {
+    background-image: linear-gradient(
+      -90deg,
+      rgba($feedback-color, 1) 20% 40%,
+      rgba($feedback-color, 0.8) 80%,
+      rgba($feedback-color, 0) 100%
+    );
+  }
+*/
+}