Re-add Google Analytics with a real GA4 measurement ID (or confirm none needed)

PR #26 removed a non-functional Google Analytics placeholder (google_analytics_id: G-XXXXXXXXXX) from lectures/_config.yml. It was never a real measurement ID, so it collected no data — removing it just stops shipping a dead config value. This issue tracks the follow-up decision: re-add analytics with a real ID, or confirm the site intentionally runs without it.

@DrDrij — do you have a GA4 measurement ID for dp.quantecon.org (or should this site report into an existing org-wide property)? If analytics are wanted, please provide the G-XXXXXXXXXX ID and we'll re-add the block; if not, we can close this as "no analytics by design".

To re-enable, add this back under html_theme_options in lectures/_config.yml (it sat just below keywords:), with the placeholder swapped for the real ID:

      analytics:
        google_analytics_id: G-XXXXXXXXXX   # replace with the real GA4 measurement ID for dp.quantecon.org

Context: split out from the tooling refresh tracked in #22; the removal landed in #26.

Start in lectures/_config.yml under html_theme_options, near keywords and the removed analytics block. First confirm whether a real GA4 measurement ID or an existing organization-wide property should be used. Done means either restoring the block with the confirmed ID or documenting that analytics are intentionally disabled.