chore: import upstream snapshot with attribution

docs/source/_static/.gitkeep

@@ -0,0 +1,6 @@
+Without this directory, Sphinx will warn:
+
+> WARNING: html_static_path entry '_static' does not exist
+
+This file exists to suppress that warning and because empty directories cannot
+be checked in to Git.

docs/source/_static/css/custom.css

@@ -0,0 +1,41 @@
+details {
+    margin-bottom: 0.75rem;
+    margin-top: 0.5rem;
+}
+
+details summary {
+    cursor: pointer;
+}
+
+details summary > * {
+    display: inline;
+}
+
+details[open] summary {
+    padding-bottom: 0.75rem;
+    border-bottom: 2px dashed #ccc;
+}
+
+details[open] {
+    border-bottom: 2px dashed #ccc;
+}
+
+h1 {
+    font-size: 2em;
+}
+
+h2 {
+    font-size: 1.5em;
+}
+
+h3 {
+    font-size: 1.17em;
+}
+
+h5 {
+    font-size: .83em;
+}
+
+h6 {
+    font-size: .75em;
+}