Set up documentation website generation and deployment (#253)

Generating the website is done by Jekyll with a theme called `just-the-docs`. 
Deployment of the website is done via GitHub Pages.

See:
- https://github.com/jekyll/jekyll
- https://github.com/just-the-docs/just-the-docs

generate-docs.sh

@@ -0,0 +1,27 @@
+#!/usr/bin/env bash
+
+set -e -u -o pipefail
+
+REPOSIORY_ROOT="$(git rev-parse --show-toplevel)"
+WEBSITE_ROOT="${REPOSIORY_ROOT}/website"
+
+generate_homepage() {
+    local homepage="${WEBSITE_ROOT}/index.md"
+
+    echo "Generating ${homepage}..."
+    cat - "${REPOSIORY_ROOT}/README.md" > "${homepage}" << EOF
+---
+layout: default
+title: Home
+nav_order: 1
+---
+EOF
+
+    local macos_compat=""
+    [[ "${OSTYPE}" == "darwin"* ]] && macos_compat="yes"
+    sed -i ${macos_compat:+".bak"} 's/src="website\//src="/g' "${homepage}"
+    sed -i ${macos_compat:+".bak"} 's/srcset="website\//srcset="/g' "${homepage}"
+}
+
+# Generate the website.
+generate_homepage