Temporary: move static files into content

During migration, while we launch Redocly with -d content, the static
files need to be in content. Eventually, when we stop using -d, we need
to move the files again.

Previously:
/assets : static assets used by templates
/img : images used in documentation (mostly)

Now:
/content/static : static assets used by templates
/img : images used in documentation (mostly)

Eventually:
/static : static assets used by templates
/docs/img : images used in documentation

content/static/js/theme-switch.js

@@ -0,0 +1,47 @@
+// Check user prefers color, toggle light/dark, save state
+// Based loosely on https://github.com/vinorodrigues/bootstrap-dark
+
+function apply_color_scheme(theme) {
+  const disable_theme = (theme == "dark") ? "light" : "dark";
+  document.documentElement.classList.add(theme)
+  document.documentElement.classList.remove(disable_theme)
+  document.documentElement.setAttribute("data-theme", theme)
+  // $("#css-toggle-btn").prop( "checked", (theme == 'dark') );
+}
+
+function auto_update_theme() {
+  const upc = window.localStorage.getItem('user-prefers-color')
+  let theme = "dark"; // Default to dark theme
+  if (!upc) {
+    // User hasn't saved a preference specifically for this site; check
+    // the browser-level preferences.
+    if (window.matchMedia &&
+        window.matchMedia("(prefers-color-scheme: light)").matches) {
+      theme = "light"
+    }
+  } else { // Follow user's saved setting.
+    theme = (upc == "light") ? "light" : "dark"
+  }
+  apply_color_scheme(theme)
+}
+
+function user_toggle_theme() {
+  const new_theme = document.documentElement.classList.contains("dark") ? "light" : "dark"
+  window.localStorage.setItem("user-prefers-color", new_theme)
+  // Animate this style switch, but not the ones that happen on page load:
+  document.body.style.transition = "background-color .2s ease"
+  apply_color_scheme(new_theme)
+}
+
+auto_update_theme()
+// update automatically if the user's theme preference changes
+if (window.matchMedia) {
+  window.matchMedia("(prefers-color-scheme: dark)").addListener( auto_update_theme )
+}
+// Note: .addListener is considered deprecated, and is supposed to be updated to
+// addEventListener("change", callback) instead; however, as recently as macOS
+// High Sierra (~2017-2018) Safari does not support addEventListener here.
+
+window.addEventListener('DOMContentLoaded', (event) => {
+  document.getElementById("css-toggle-btn").onclick = user_toggle_theme
+})