chore: add the Google Search Console verification file

Verifies ownership of https://cpp-linter.github.io/ in Search Console. The
Google Analytics method cannot be used: Material injects gtag.js from a
script at runtime, and the verifier only looks for it in the served <head>.
Google requires the file to stay in place after verification.

Author: shenxianpeng

docs/google7a7018130a36ef4d.html

@@ -0,0 +1 @@
+google-site-verification: google7a7018130a36ef4d.html