docs: add some docs (#14)

Author: fengjiachun

docs/global-options.md

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+# Global Options (System properties / Java -Dxxx)
+
+| Name                               | Description                                                                                                                                                        |
+|:-----------------------------------|:-------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| greptimedb.use_os_signal           | Whether or not to use OS Signal, SDK listens for SIGUSR2 signals by default and can outputs some information. This is helpful when troubleshooting complex issues. |
+| greptimedb.signal.out_dir          | Signal handler can output to the specified directory, default is the process start directory.                                                                      |
+| greptimedb.available_cpus          | Specify the number of available cpus, the default is to use the full number of cpus of the current environment.                                                    |
+| greptimedb.reporter.period_minutes | Metrics reporter timed output period, default 30 minutes.                                                                                                          |
+| greptimedb.read.write.rw_logging   | Whether to print logs for each read/write operation, default off.                                                                                                  |

docs/magic-tools.md

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+# Magic Tools
+
+### How to use `kill -s SIGUSR2 $pid`
+
+The first time you execute `kill -s SIGUSR2 $pid` you will see the following help messages on
+the log output, including:
+
+- Turn on/off the output of the condensed version of the read/write log.
+- Turn on/off limiter
+- Export in-memory metrics and memory state information of important objects to a local file
+
+### Just follow the help information
+
+```text
+- -- GreptimeDB Signal Help --
+-     Signal output dir: /Users/xxx
+- 
+-     How to open or close read/write log(The second execution means close):
+-       [1] `cd /Users/xxx`
+-       [2] `touch rw_logging.sig`
+-       [3] `kill -s SIGUSR2 $pid`
+-       [4] `rm rw_logging.sig`
+- 
+- 
+-     How to open or close rpc limiter(The second execution means close):
+-       [1] `cd /Users/xxx`
+-       [2] `touch rpc_limit.sig`
+-       [3] `kill -s SIGUSR2 $pid`
+-       [4] `rm rpc_limit.sig`
+- 
+-     How to get metrics and display info:
+-       [1] `cd /Users/xxx`
+-       [2] `rm *.sig`
+-       [3] `kill -s SIGUSR2 $pid`
+- 
+-     The file signals that is currently open:
+- 
+- Displaying GreptimeDB clients triggered by signal: USR2 to file: /Users/xxx/greptimedb_client_display.log.2024-01-09_16-28-38.
+- Printing GreptimeDB client metrics triggered by signal: USR2 to file: /Users/xxx/greptimedb_client_metrics.log.2024-01-09_16-28-38.
+```