[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/_rpc/ns-miner.md

@@ -0,0 +1,72 @@
+---
+title: miner Namespace
+sort_key: C
+---
+
+The `miner` API allows you to remote control the node's mining operation and set various
+mining specific settings.
+
+* TOC
+{:toc}
+
+### miner_getHashrate
+
+Get your hashrate in H/s (Hash operations per second).
+
+| Client  | Method invocation                                           |
+|:--------|-------------------------------------------------------------|
+| Console | `miner.getHashrate()`                                       |
+| RPC     | `{"method": "miner_getHashrate", "params": []}`             |
+
+### miner_setExtra
+
+Sets the extra data a miner can include when miner blocks. This is capped at
+32 bytes.
+
+| Client  | Method invocation                                  |
+|:--------|----------------------------------------------------|
+| Go      | `miner.setExtra(extra string) (bool, error)`       |
+| Console | `miner.setExtra(string)`                           |
+| RPC     | `{"method": "miner_setExtra", "params": [string]}` |
+
+### miner_setGasPrice
+
+Sets the minimal accepted gas price when mining transactions. Any transactions that are
+below this limit are excluded from the mining process.
+
+| Client  | Method invocation                                     |
+|:--------|-------------------------------------------------------|
+| Go      | `miner.setGasPrice(number *rpc.HexNumber) bool`       |
+| Console | `miner.setGasPrice(number)`                           |
+| RPC     | `{"method": "miner_setGasPrice", "params": [number]}` |
+
+### miner_start
+
+Start the CPU mining process with the given number of threads and generate a new DAG
+if need be.
+
+| Client  | Method invocation                                   |
+|:--------|-----------------------------------------------------|
+| Go      | `miner.Start(threads *rpc.HexNumber) (bool, error)` |
+| Console | `miner.start(number)`                               |
+| RPC     | `{"method": "miner_start", "params": [number]}`     |
+
+### miner_stop
+
+Stop the CPU mining operation.
+
+| Client  | Method invocation                            |
+|:--------|----------------------------------------------|
+| Go      | `miner.Stop() bool`                          |
+| Console | `miner.stop()`                               |
+| RPC     | `{"method": "miner_stop", "params": []}`     |
+
+### miner_setEtherbase
+
+Sets the etherbase, where mining rewards will go.
+
+| Client  | Method invocation                                           |
+|:--------|-------------------------------------------------------------|
+| Go      | `miner.SetEtherbase(common.Address) bool`                   |
+| Console | `miner.setEtherbase(address)`                               |
+| RPC     | `{"method": "miner_setEtherbase", "params": [address]}`     |