[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/server.md

@@ -0,0 +1,84 @@
+---
+title: JSON-RPC Server
+sort_key: A
+---
+
+Geth supports all standard web3 JSON-RPC APIs. You can find documentation for
+these APIs on the [Ethereum Wiki JSON-RPC page](web3-rpc).
+
+JSON-RPC is provided on multiple transports. Geth supports JSON-RPC over HTTP,
+WebSocket and Unix Domain Sockets. Transports must be enabled through
+command-line flags.
+
+Ethereum JSON-RPC APIs use a name-space system. RPC methods are grouped into
+several categories depending on their purpose. All method names are composed of
+the namespace, an underscore, and the actual method name within the namespace.
+For example, the `eth_call` method resides in the `eth` namespace.
+
+Access to RPC methods can be enabled on a per-namespace basis. Find
+documentation for individual namespaces in the sidebar.
+
+### HTTP Server
+
+To enable the HTTP server, use the `--rpc` flag.
+
+    geth --rpc
+
+By default, geth accepts connections from the loopback interface (127.0.0.1).
+The default listening port is 8545. You can customize address and port using the
+`--rpcport` and `--rpcaddr` flags.
+
+    geth --rpc --rpcport 3334
+
+JSON-RPC method namespaces must be whitelisted in order to be available through
+the HTTP server. The default whitelist allows access to the "eth" and "shh"
+namespaces. To enable access to other APIs like account management ("personal")
+and debugging ("debug"), they must be configured via the `--rpcapi` flag. We do
+not recommend enabling such APIs over HTTP, however, since access to these
+methods increases the attack surface.
+
+    geth --rpc --rpcapi personal,eth,net,web3b
+
+Since the HTTP server is reachable from any local application, additional
+protection is built into the server to prevent misuse of the API from web pages.
+If you want enable access to the API from a web page, you must configure the
+server to accept Cross-Origin requests with the `--rpccorsdomain` flag.
+
+Example: if you want to use [Remix](remix) with geth, allow requests from the
+remix domain.
+
+    geth --rpc --rpccorsdomain https://remix.ethereum.org
+
+Use `--rpccorsdomain '*'` to enable access from any origin.
+
+### WebSocket Server
+
+Configuration of the WebSocket endpoint is similar to the HTTP transport. To
+enable WebSocket access, use `--ws` flag. The default WebSocket port is 8546.
+The `--wsaddr`, `--wsport` and `--wsapi` flags can be used to customize settings
+for the WebSocket server.
+
+    geth --ws --wsport 3334 --wsapi eth,net,web3
+
+Cross-Origin request protection also applies to the WebSocket server. Use the
+`--wsorigins` flag to allow access to the server from web pages:
+
+    geth --ws --wsorigins http://myapp.example.com
+
+As with `--rpccorsdomain`, using `--wsorigins '*'` allows access from any origin.
+
+### IPC Server
+
+JSON-RPC APIs are also provided on a UNIX domain socket. This server is enabled
+by default and has access to all JSON-RPC namespaces.
+
+The listening socket is placed into the data directory by default. On Linux, the
+default location of the geth socket is
+
+    ~/.ethereum/geth.ipc
+
+You can configure the location of the socket using the `--ipcpath` flag. IPC can
+be disabled using the `--ipcdisable` flag.
+
+[web3-rpc]: https://github.com/ethereum/wiki/wiki/JSON-RPC
+[remix]: https://remix.ethereum.org