docs: add page=README.md param to Scarf pixel for GitHub obfuscation

Per Scarf's GitHub obfuscation guidance
(https://docs.scarf.sh/web-traffic/#obfuscation-on-github), GitHub's
camo image proxy rewrites image URLs to camo.githubusercontent.com,
which both strips the referrer and obscures which page is being
viewed. Adding the `page=README.md` query param tags the pixel
impression with its source page so it still attributes correctly.

Author: Ilia

README.md

@@ -17,7 +17,7 @@ This repository contains a Dockerized version of the [Pact Broker][pact-broker].
 [![size: arm64](https://badgen.net/docker/size/pactfoundation/pact-broker/latest-multi/arm64?icon=docker&label=size%3Aarm64)](https://hub.docker.com/r/pactfoundation/pact-broker)
 [![size: arm](https://badgen.net/docker/size/pactfoundation/pact-broker/latest-multi/arm?icon=docker&label=size%3Aarm)](https://hub.docker.com/r/pactfoundation/pact-broker)
 
-<img referrerpolicy="no-referrer-when-downgrade" src="https://static.scarf.sh/a.png?x-pxid=f0adca49-1631-4f0b-8914-366ea390b5c8" />
+<img referrerpolicy="no-referrer-when-downgrade" src="https://static.scarf.sh/a.png?x-pxid=f0adca49-1631-4f0b-8914-366ea390b5c8&page=README.md" />
 
 ## In a hurry?