Fix status badge for documentation (#7304)

Documentation has been moved to GitHub pages and no longer use readthedocs. Updated the README status badge for docs with correct link

Signed-off-by: Chaminda Divitotawela <[email protected]>

README.md

@@ -1,6 +1,6 @@
 # Besu Ethereum Client
  [![CircleCI](https://circleci.com/gh/hyperledger/besu/tree/main.svg?style=svg)](https://circleci.com/gh/hyperledger/besu/tree/main)
- [![Documentation Status](https://readthedocs.org/projects/hyperledger-besu/badge/?version=latest)](https://besu.hyperledger.org/en/latest/?badge=latest)
+ [![Documentation](https://img.shields.io/github/actions/workflow/status/hyperledger/besu-docs/publish-main-docs.yml?branch=main&label=docs)](https://github.com/hyperledger/besu-docs/actions/workflows/publish-main-docs.yml)
  [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/3174/badge)](https://bestpractices.coreinfrastructure.org/projects/3174)
  [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://github.com/hyperledger/besu/blob/main/LICENSE)
  [![Discord](https://img.shields.io/discord/905194001349627914?logo=Hyperledger&style=plastic)](https://discord.gg/hyperledger)