[DOCS] massive documentation update (#20229)

This PR:

- reorganizes all documentation pages so they live in the right category
- removes lots of legacy docs
- contains many improvements to active documentation pages

Geth user documentation is now spread across five major categories:

- Install and Build: installation and compile instructions
- Using Geth: this is for pages about general geth usage.
- For dApp Developers: this is for programming guides and functionality specific
   to dapp development. All the dev guides for mobile framework and Go APIs live here.
- JSON-RPC APIs: this has its own section because there is now a sub-page for
   every name space. I have also added an overview text that explains how to set
   up the API servers.
- For Geth Developers: this is for geth contributors

docs/_whisper/Achieving-Darkness.md

@@ -1,6 +1,8 @@
 ---
 title: Achieving Darkness
+sort_key: B
 ---
+
 Whisper is designed to be a building block in the next generation of unstoppable ÐApps. It was designed to provide resilience and privacy at considerable expense. At its most secure mode of operation Whisper can theoretically deliver complete darkness. Whisper should also allow the users to configure the level of privacy (how much information it leaks concerning the ÐApp content and ultimately, user activities) as a trade-off for performance. In this article we will discuss the strategy of achieving complete darkness.
 
 ### Adversary

docs/_whisper/Diagnostic-Tool-wnode.md

@@ -1,7 +1,7 @@
 ---
 title: Diagnostic tool wnode
+sort_key: B
 ---
-# Wnode
 
 Wnode (whisper node) is a command-line diagnostic tool. It does not have a nice user interface, because its main purpose is diagnostic, and it's meant to be very light-weight rather than beautiful. Wnode might be used for different purposes, including:
 

docs/_whisper/How-to-Whisper.md

@@ -1,9 +1,11 @@
 ---
 title: How to Whisper
+sort_key: B
 ---
+
 Whisper is a pure identity-based messaging system. Whisper provides a low-level (non-application-specific) but easily-accessible API without being based upon or prejudiced by the low-level hardware attributes and characteristics, particularly the notion of singular endpoints.
 
-This tutorial assumes you've read [p2p 101](../developers/peer-to-peer). If you haven't read it I suggest you read it. This tutorial will guide you to setting up a full p2p server with whisper capabilities.
+This tutorial will guide you to setting up a full p2p server with whisper capabilities.
 
 Let's quickly cover some of whisper's basic functionality and discuss it in greater detail later.
 
@@ -97,4 +99,4 @@ func main() {
 
 	select {}
 }
-```
+```

docs/_whisper/Whisper-Overview.md

@@ -1,5 +1,6 @@
 ---
 title: Overview
+sort_key: A
 ---
 
 Whisper is a pure identity-based messaging system. Whisper provides a simple low-level API without being based upon or influenced by the low-level hardware attributes and characteristics. Peer-to-peer communication between the nodes of Whisper network uses the underlying [ÐΞVp2p Wire Protocol](https://github.com/ethereum/wiki/wiki/%C3%90%CE%9EVp2p-Wire-Protocol). Whisper was not designed to provide a connection-oriented system, nor for simply delivering data between a pair of particular network endpoints. However, this might be necessary in some very specific cases (e.g. delivering the expired messages in case they were missed), and Whisper protocol will accommodate for that. Whisper is designed for easy and efficient broadcasting, and also for low-level asynchronous communications. It is designed to be a building block in next generation of unstoppable ÐApps. It was designed to provide resilience and privacy at considerable expense. At its most secure mode of operation, Whisper can theoretically deliver 100% darkness. Whisper should also allow the users to configure the level of privacy (how much information it leaks concerning the ÐApp content and ultimately, user activities) as a trade-off for performance. 

docs/_whisper/Whisper-js-example.md

@@ -1,5 +1,6 @@
 ---
 title: Whisper JavaScript example
+sort_key: B
 ---
 
 [This link](https://github.com/gballet/whisper-chat-example) contains a full-fledged example of how to use Whisper in a small chat application.

docs/_whisper/Whisper-v6-RPC-API.md

@@ -1,5 +1,6 @@
 ---
 title: Whisper RPC API 6.0
+sort_key: C
 ---
 
 This is the proposed API for whisper v6.
@@ -782,4 +783,4 @@ curl -X POST --data '{"jsonrpc":"2.0","method":"shh_post","params":[{
   "jsonrpc": "2.0",
   "result": true
 }
-```
+```