fix(docs): add index.md to docs in the root

Reading Mkdocs documentation about index file: Mkdocs will build any
files with an .md extension. If you have a README.md, it will be
transformed into an index.md automatically.

That doesn't happen automatically.

docs/index.md

@@ -0,0 +1,15 @@
+## [About](about/)
+
+Learn more about Argo and why it came about and the general philosophy behind it.
+
+## [Architecture](dev/architecture.md)
+
+Explains how the code is organized and why it is organized in that way.
+
+## [Workflow](dev/workflow.md)
+
+Describes the recommended steps to develop and to build a new release.
+
+## [Views](views/)
+
+Explains views details.