feat(docs): add Erlang cheat sheet (#673)

Create an Erlang备忘清单 in markdown format to document common commands and
operations for Erlang programming, including入门, 代码结构, 常用内置函数 (BIFs),
并发编程, 错误处理, 分布式编程, OTP框架, 和测试.

This cheat sheet aims to provide a quick reference for Erlang developers and
learners, summarizing key points and examples for various aspects of the
language.

docs/erlang.md

@@ -0,0 +1,251 @@
+Erlang 备忘清单
+===
+
+Erlang 是一种用于构建并发、分布式和容错系统的编程语言。以下是一些常用的命令和操作。
+
+入门
+---
+
+### 启动 Erlang Shell
+
+```shell
+erl
+```
+
+### 编译代码
+
+```shell
+# 在 Erlang Shell 中编译
+c(module).
+# 在命令行中编译
+erlc module.erl
+```
+
+### 运行代码
+
+```shell
+# 在 Erlang Shell 中运行
+module:function().
+# 从命令行运行
+erl -noshell -s module function -s init stop
+```
+
+### 退出 Erlang Shell
+
+```shell
+q().
+```
+
+代码结构
+---
+
+### 模块定义
+
+```erlang
+-module(module_name).
+-export([function_name/arity, ...]).
+
+function_name(Args) ->
+    % Function body.
+    Result.
+```
+
+### 导出函数
+
+```erlang
+-export([function1/0, function2/1]).
+```
+
+### 注释
+
+```erlang
+% 单行注释
+```
+
+常用内置函数 (BIFs)
+---
+
+### 列表操作
+
+```erlang
+lists:append(List1, List2).
+lists:map(Function, List).
+lists:filter(Function, List).
+lists:foldl(Function, Acc, List).
+```
+
+### 元组操作
+
+```erlang
+element(N, Tuple).
+setelement(N, Tuple, Value).
+tuple_size(Tuple).
+```
+
+### 字符串操作
+
+```erlang
+string:len(String).
+string:concat(String1, String2).
+string:tokens(String, Delimiters).
+```
+
+### 文件操作
+
+```erlang
+file:read_file(Filename).
+file:write_file(Filename, Data).
+file:delete(Filename).
+```
+
+并发编程
+---
+
+### 创建进程
+
+```erlang
+Pid = spawn(Module, Function, Args).
+```
+
+### 发送消息
+
+```erlang
+Pid ! Message.
+```
+
+### 接收消息
+
+```erlang
+receive
+    Pattern1 -> Actions1;
+    Pattern2 -> Actions2;
+    ...
+end.
+```
+
+### 链接进程
+
+```erlang
+link(Pid).
+unlink(Pid).
+```
+
+### 监控进程
+
+```erlang
+MonitorRef = erlang:monitor(process, Pid).
+erlang:demonitor(MonitorRef).
+```
+
+错误处理
+---
+
+### 捕获异常
+
+```erlang
+try Expression of
+    Pattern -> Result
+catch
+    Class:Reason -> Handler
+end.
+```
+
+### 常见异常类型
+
+- `throw`
+- `error`
+- `exit`
+
+分布式编程
+---
+
+### 启动分布式节点
+
+```shell
+erl -name nodename@hostname -setcookie Cookie
+```
+
+### 连接节点
+
+```erlang
+net_adm:ping(Node).
+```
+
+### 发送消息到远程节点
+
+```erlang
+{remote_process, 'remote_node@host'} ! Message.
+```
+
+OTP 框架
+---
+
+### 定义 GenServer
+
+```erlang
+-module(my_gen_server).
+-behaviour(gen_server).
+
+-export([start_link/0, init/1, handle_call/3, handle_cast/2, handle_info/2, terminate/2, code_change/3]).
+
+start_link() ->
+    gen_server:start_link({local, ?MODULE}, ?MODULE, [], []).
+
+init([]) ->
+    {ok, #state{}}.
+
+handle_call(Request, From, State) ->
+    {reply, Reply, State}.
+
+handle_cast(Msg, State) ->
+    {noreply, State}.
+
+handle_info(Info, State) ->
+    {noreply, State}.
+
+terminate(Reason, State) ->
+    ok.
+
+code_change(OldVsn, State, Extra) ->
+    {ok, State}.
+```
+
+### 使用 GenServer
+
+```erlang
+gen_server:start_link({local, Name}, Module, Args, Options).
+gen_server:call(ServerRef, Request).
+gen_server:cast(ServerRef, Msg).
+```
+
+测试
+---
+
+### 编写 EUnit 测试
+
+```erlang
+-module(module_name_tests).
+-include_lib("eunit/include/eunit.hrl").
+
+simple_test() ->
+    ?assertEqual(Expected, Actual).
+
+complex_test_() ->
+    [
+        {"Test case 1", ?_assertEqual(Expected1, Actual1)},
+        {"Test case 2", ?_assertEqual(Expected2, Actual2)}
+    ].
+```
+
+### 运行 EUnit 测试
+
+```shell
+# 在命令行中运行
+erl -eval "eunit:test(module_name)" -s init stop
+```
+
+另见
+---
+
+- [Erlang 官方文档](https://www.erlang.org/docs)
+- [Erlang 编程书籍](https://www.erlang.org/books)