[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/_developers/Code-Review-Guidelines.md

@@ -1,6 +1,8 @@
 ---
 title: Code Review Guidelines
+sort_key: B
 ---
+
 The only way to get code into go-ethereum is to send a pull request. Those pull requests
 need to be reviewed by someone. This document is a guide that explains our expectations
 around PRs for both authors and reviewers.