[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/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 {}
 }
-```
+```