Initial mkdocs setup

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docs/index.md

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+# Welcome to urfave/cli
+
+[![GoDoc](https://godoc.org/github.com/urfave/cli?status.svg)](https://pkg.go.dev/github.com/urfave/cli/v2)
+[![codebeat](https://codebeat.co/badges/0a8f30aa-f975-404b-b878-5fab3ae1cc5f)](https://codebeat.co/projects/github-com-urfave-cli)
+[![Go Report Card](https://goreportcard.com/badge/urfave/cli)](https://goreportcard.com/report/urfave/cli)
+[![codecov](https://codecov.io/gh/urfave/cli/branch/main/graph/badge.svg)](https://codecov.io/gh/urfave/cli)
+
+`urfave/cli` is a simple, fast, and fun package for building command line apps in Go. The
+goal is to enable developers to write fast and distributable command line applications in
+an expressive way.
+
+These are the guides for each major supported version:
+
+- [`v2`](./v2/)
+- [`v1`](./v1/)
+
+In addition to the version-specific guides, these other documents are available:
+
+- [`CONTRIBUTING`](./CONTRIBUTING/)
+- [`RELEASING`](./RELEASING/)