doc: front page of etcd3 doc

Documentation/docs.md

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+# Documentation
+
+etcd is a distributed key-value store designed to reliably and quickly preserve and provide access to critical data. It enables reliable distributed coordination through distributed locking, leader elections, and write barriers. An etcd cluster is intended for high availability and permanent data storage and retrieval.
+
+## Getting started
+
+New to etcd? Start by [downloading and building][download_build] etcd!
+
+## Developing with etcd
+
+Want to develop applications with etcd? Start by setting up a local cluster!
+
+ - Setting up local clusters
+ - Interacting with etcd
+ - API references
+
+## Operating etcd clusters
+
+Want to operate etcd clusters? Start by setting up a cluster on multiple machines!
+
+ - Setting up clusters
+ - Configuration
+ - Security
+ - Monitoring
+ - Maintenance
+ - Disaster recovery
+ - Performance
+
+## Learning
+
+Want to learn more about the concepts and internals behind etcd? Read the following pages!
+
+ - Why etcd
+ - Concepts
+ - Internals
+ - Glossary
+
+## Upgrading and compatibility
+
+## Troubleshooting
+
+[download_build]: dl_build.md