Add Plausible Analytics

[StanFromIreland]

As suggested in python/devguide#1749 (review).

[JacobCoffee]

<script defer data-domain="blog.python.org" src="https://analytics.python.org/js/script.hash.outbound-links.js"></script>
<script>window.plausible = window.plausible || function() { (window.plausible.q = window.plausible.q || []).push(arguments) }</script>

(preferred) for 404s, hashed paths, and outbound link tracking

<script defer data-domain="blog.python.org" src="https://analytics.python.org/js/script.outbound-links.js"></script>

for just outbound links

<script defer data-domain="blog.python.org" src="https://analytics.python.org/js/script.js"></script>

for just the basics

[StanFromIreland]

Updated to the preferred.