Add analytics to Cabal User Guide

**Overview:**

Add analytics tracking to Cabal User Guide via ReadTheDocs and Google Analytics integration (see
https://docs.readthedocs.io/en/stable/guides/google-analytics.html).

**Rationale:**

- Determining which locales access the docs the most often
- Benefit: Informs us if it would be useful to invest in an i18n version of the docs, and if so which languages
- Detecting dead links and duplicate pages, info missing from sitemap, etc.
- Benefit: Improves SEO, letting users find what they're looking for more easily
- Bounce rate on individual pages to give insight into when users aren't finding what they're looking for on a page
- Benefit: Informs us where attention should be focused in the existing docs
- Referrer
- Benefits: Lets us determine if there's other locations that we want links to the docs to be visible, but is being missed (ex. we expect users to get to the user guide from https://www.haskell.org/cabal/, but we don't know if users are actually finding it on the page, so we could try out different placements and formats)

**Privacy:**

To address privacy concerns, from the [ReadTheDocs analytics integration](https://docs.readthedocs.io/en/stable/guides/google-analytics.html): "Read the Docs takes some extra precautions with analytics to protect user privacy. As a result, users with Do Not Track enabled will not be counted for the purpose of analytics." In addition, User-Id tracking (which allows cross-device session attribution) is disabled.

Start with the linked Read the Docs Google Analytics integration guide and inspect the Cabal User Guide's Read the Docs configuration. Determine where the integration is configured, then verify that analytics is enabled while the stated privacy protections remain in effect.

Written by the indexing model from the issue text.