Document detect_glib_libc() and detect_musl_libc() functions (#3590)

* Document detect_glib_libc() and detect_musl_libc() functions

This documents the functions added in conan-io/conan#15683.

* Document the detect_libc() function

* Revert "Document detect_glib_libc() and detect_musl_libc() functions"

This reverts commit b4f58e8.

* Mark detect_libc as experimental

reference/config_files/profiles.rst

@@ -664,6 +664,7 @@ the associated runtime, you can use:
 
     - **`detect_os()`**: returns operating system as a string (e.g., "Windows", "Macos").
     - **`detect_arch()`**: returns system architecture as a string (e.g., "x86_64", "armv8").
+    - **`detect_libc(ldd="/usr/bin/ldd")`**: **experimental** returns a tuple with the name (e.g., "gnu", "musl") and version (e.g., "2.39", "1.2.4") of the C library.
     - **`detect_libcxx(compiler, version, compiler_exe=None)`**: returns C++ standard library as a string (e.g., "libstdc++", "libc++").
     - **`default_msvc_runtime(compiler)`**: returns tuple with runtime (e.g., "dynamic") and its version (e.g., "v143").
     - **`default_cppstd(compiler, compiler_version)`**: returns default C++ standard as a string (e.g., "gnu14").