Docs: prep for upcoming RTD build changes

Read the Docs is changing their build process. Some docs/conf.py
manipulation they used to do automatically now must be manually
included.

https://about.readthedocs.com/blog/2024/07/addons-by-default/

Related changes:

* Drop our version-alert.js. RTD's new "addons" includes
  equivalent warning on unreleased versions.

* Integrate Google Analytics via sphinxcontrib-googleanalytics
  when GOOGLE_ANALYTICS_ID is set in docs build environment.

docs/_static/version-alert.js

@@ -1,38 +0,0 @@
-"use strict";
-
-// Similar to https://github.com/readthedocs/readthedocs.org/blob/5.2.3/readthedocs/core/static-src/core/js/doc-embed/version-compare.js
-// but adds admonition for the "latest" version -- which is (unreleased) main branch.
-
-function warnOnLatestVersion() {
-  // The warning text and link is really specific to RTD hosting,
-  // so we can just check their global to determine version:
-  if (!window.READTHEDOCS_DATA || window.READTHEDOCS_DATA.version !== "latest") {
-    return; // not latest, or not on RTD
-  }
-
-  var warning = document.createElement("div");
-  warning.setAttribute("class", "admonition danger");
-  warning.innerHTML =
-    "<p class='first admonition-title'>Note</p> " +
-    "<p class='last'> " +
-    "This document is for an <strong>unreleased development version</strong>. " +
-    "Documentation is available for the <a href='/en/stable/'>current stable release</a>, " +
-    "or for older versions through the &ldquo;v:&rdquo; menu at bottom left." +
-    "</p>";
-  warning.querySelector("a").href = window.location.pathname.replace(
-    "/latest",
-    "/stable"
-  );
-
-  var parent =
-    document.querySelector("div.body") ||
-    document.querySelector("div.document") ||
-    document.body;
-  parent.insertBefore(warning, parent.firstChild);
-}
-
-if (document.readyState === "loading") {
-  document.addEventListener("DOMContentLoaded", warnOnLatestVersion);
-} else {
-  warnOnLatestVersion();
-}