Docs: Invert logo color for dark theme in README (#76012)

docs: Invert logo color for dark theme in README

Author: Sea-n

README.md

@@ -1,4 +1,5 @@
-![Grafana](docs/logo-horizontal.png)
+![Grafana Logo (Light)](docs/logo-horizontal.png#gh-light-mode-only)
+![Grafana Logo (Dark)](docs/logo-horizontal-dark.png#gh-dark-mode-only)
 
 The open-source platform for monitoring and observability