docs: add index page for docs site

docs/index.md

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+# Diagrams
+
+## Diagrams as Code
+
+- [Try it out](getting-started/installation.md)
+- [Show examples](getting-started/examples.md)
+
+
+## About Diagrams
+
+Diagrams lets you draw the cloud system architecture **in Python code**.
+
+It was born for **prototyping** a new system architecture without any design tools. You can also describe or visualize the existing system architecture as well.
+
+Diagram as Code allows you to **track** the architecture diagram changes in any **version control** system.
+
+Diagrams currently supports six major providers: AWS, Azure, GCP, Kubernetes, Alibaba Cloud and Oracle Cloud. It now also supports On-Premise nodes as well as Programming Languages and Frameworks.
+
+![event-processing-code](img/event_processing_code.png)