docs: add docusaurus badge for documentation (#1070)

README.md

@@ -4,6 +4,7 @@
 [![codecov](https://codecov.io/gh/anitab-org/mentorship-backend/branch/develop/graph/badge.svg)](https://codecov.io/gh/anitab-org/mentorship-backend)
 [![project chat](https://img.shields.io/badge/zulip-join_chat-brightgreen.svg)](https://anitab-org.zulipchat.com/#narrow/stream/222534-mentorship-system)
 [![Python 3.x](https://img.shields.io/badge/python-3.x-blue.svg)](https://www.python.org/downloads/)
+[![Docs](https://img.shields.io/badge/documentation-mentorship--backend-blue.svg)](https://anitab-org.github.io/mentorship-backend)
 
 
 Mentorship System is an application that allows women in tech to mentor each other, on career development topics, through 1:1 relations for a certain period.