Documentation Structure Change and Cleanup (#2808)

* Update docs/sdk/clients.md
* organize ADR directory like tendermint
* docs: move spec-proposals into spec/
* remove lotion, moved to website repo
* move getting-started to cosmos-hub, and voyager to website
* docs: move lite/ into clients/lite/
* move introduction/ content to website repo
* move resources/ content to website repo
* mv sdk/clients.md to clients/clients.md
* mv validators to cosmos-hub/validators
* move deprecated sdk/ content to _attic
* sdk/modules.md is duplicate with modules/README.md
* consolidate remianing sdk/ files into a single sdk.md
* move examples/ to docs/examples/
* mv docs/cosmos-hub to docs/gaia
* Add keys/accounts section to localnet docs

docs/README.md

@@ -1,24 +1,38 @@
-# Welcome to the Cosmos Docs!
+# Welcome to the SDK Docs!
 
-![cosmonaut reading the cosmos docs in space](./graphics/cosmos-docs.jpg)
+Welcome to the SDK docs!
 
-Cosmos is a decentralized network of independent parallel blockchains, each powered by classical BFT consensus algorithms like Tendermint.
 
-The first blockchain in the Cosmos Network is the Cosmos Hub, whose native token is the Atom. Cosmos is a permission-less network, meaning that anybody can build a blockchain on it.
+![cosmonaut reading the cosmos docs in space](./cosmos-docs.jpg)
 
-Cosmos can interoperate with multiple other applications and cryptocurrencies. By creating a new zone, you can plug any blockchain system into the Cosmos hub and pass tokens back and forth between those zones, without the need for an intermediary.
+## Learn the SDK
 
-## Quick Start
+### SDK Intro
 
-- [Getting started with the SDK](./sdk/core/intro.md)
-- [SDK Examples](../examples)
-- [Join the testnet](./getting-started/join-testnet.md#run-a-full-node)
+If you are a newcomer and would like to learn more about the Cosmos SDK, this **[intro](./intro/README.md)** is a good starting place.
+
+### SDK tutorial
+
+If you like to learn by doing, you can follow the **[SDK application tutorial](https://github.com/cosmos/sdk-application-tutorial)**. It showcases how to build an SDK-based blockchain from scratch, and teaches you about the basic principles the SDK in the process.
+
+## Use the SDK
+
+The following sections contain the information you need if you want to build a fully-functional SDK-based blockchain:
+
+>*NOTE*: We are currently working on improving the docs. Some info might be missing. If that is the case, try the Cosmos [Forum](https://forum.cosmos.network). Failing that, [open an issue](https://github.com/cosmos/cosmos-sdk/issues/new).
+
+- [Introduction](./intro/README.md): Contains introductory high-level material on the Cosmos SDK.
+- [Gaia](./gaia/README.md): Contains all documentation related to the gaia application (current name for the Cosmos-Hub).
+- [Clients](./clients/README.md): Documentation about SDK clients like the SDK Command-Line interface and the SDK Light-client.
+- [Specifications](./spec/README.md): Contains SDK and modules specifications.
+
+If you are reading this on the Cosmos Website, please know that you can find more information on [github](https://github.com/cosmos/cosmos-sdk/tree/develop/docs). Also if you find any issues with the documentation please [*open a Pull Request*](https://github.com/cosmos/cosmos-sdk/compare?expand=1), or at least [*open an Issue*](https://github.com/cosmos/cosmos-sdk/issues/new) to update the docs!
+
+## Join the public testnet for the Cosmos Hub
+
+To install the latest version of the `gaia` application and join the public testnet, **click [here](./gaia/README.md#join-the-cosmos-hub-public-testnet)**
 
 ## Edit the Documentation
 
 See [this file](./DOCS_README.md) for details of the build process and
 considerations when making changes.
-
-## Version
-
-This documentation is built from the following commit: