docs: add built-in profiler

docs/profiling.md

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+# Profiling SFTPGo
+
+The built-in profiler lets you collect CPU profiles, traces, allocations and heap profiles that allow to identify and correct specific bottlenecks.
+You can enable the built-in profiler using the `--profiler` command flag.
+
+Profiling data are exposed via HTTP/HTTPS in the format expected by the [pprof](https://github.com/google/pprof/blob/master/doc/README.md) visualization tool. You can find the index page at the URL `/debug/pprof/`.
+
+The following profiles are available, you can obtain them via HTTP GET requests:
+
+- `allocs`, a sampling of all past memory allocations
+- `block`, stack traces that led to blocking on synchronization primitives
+- `goroutine`, stack traces of all current goroutines
+- `heap`, a sampling of memory allocations of live objects. You can specify the `gc` GET parameter to run GC before taking the heap sample
+- `mutex`, stack traces of holders of contended mutexes
+- `profile`, CPU profile. You can specify the duration in the `seconds` GET parameter. After you get the profile file, use the `go tool pprof` command to investigate the profile
+- `threadcreate`, stack traces that led to the creation of new OS threads
+- `trace`, a trace of execution of the current program. You can specify the duration in the `seconds` GET parameter. After you get the trace file, use the `go tool trace` command to investigate the trace
+
+Let's see some examples:
+
+- download a 30 seconds CPU profile from the URL `/debug/pprof/profile?seconds=30`
+- download a sampling of memory allocations of live objects from the URL `/debug/pprof/heap?gc=1`
+- download a sampling of all past memory allocations from the URL `/debug/pprof/allocs`