docs: Add badge for "docs-publish" GitHub Action

Add a dynamically generated badge to show the current state of the
"docs-publish" GitHub Action [for the "main" branch].

Here is some interesting discussion on whether it would be possible to
make it update dynamically depending on the branch you're viewing
README.md from:

https://docs.github.com/en/actions/monitoring-and-troubleshooting-workflows/monitoring-workflows/adding-a-workflow-status-badge#using-the-branch-parameter
https://stackoverflow.com/questions/18673694/referencing-current-branch-in-github-readme-md

Signed-off-by: Vangelis Koukis <vkoukis@gmail.com>

README.md

@@ -7,6 +7,7 @@
 
 [![License: CC BY-SA 4.0](https://img.shields.io/badge/License-CC%20BY--SA%204.0-lightgrey.svg)](https://creativecommons.org/licenses/by-sa/4.0/)
 [![License: Apache 2.0](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://www.apache.org/licenses/LICENSE-2.0)
+[![GitHub Action: Documentation](https://github.com/vkoukis/circuit-explorers/actions/workflows/docs-publish.yaml/badge.svg)](https://github.com/vkoukis/circuit-explorers/actions/workflows/docs-publish.yaml)
 
 </div>