Add a badge indicating status of sphinx generation (#745)

Adds a badge to the README that indicates if the generate-docs workflow succeeded for `master`.

Author: Diptorup Deb

README.md

@@ -2,6 +2,7 @@
 [![Imports: isort](https://img.shields.io/badge/%20imports-isort-%231674b1?style=flat&labelColor=ef8336)](https://pycqa.github.io/isort/)
 [![pre-commit](https://img.shields.io/badge/pre--commit-enabled-brightgreen?logo=pre-commit&logoColor=white)](https://github.com/pre-commit/pre-commit)
 [![Coverage Status](https://coveralls.io/repos/github/IntelPython/dpctl/badge.svg?branch=master)](https://coveralls.io/github/IntelPython/dpctl?branch=master)
+![Generate Documentation](https://github.com/IntelPython/dpctl/actions/workflows/generate-docs.yml/badge.svg?branch=master)
 
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