Documentation: add stdbit.h reference

Signed-off-by: Arjav Patel <arjav1528@gmail.com>

Author: arjav1528

Documentation/components/libs/libc/index.rst

@@ -51,6 +51,7 @@ functions.  So we have::
   sched     - sched.h
   search    - search.h
   semaphore - semaphore.h
+  stdbit    - stdbit.h (optional C23)
   stdio     - stdio.h
   stdlib    - stdlib.h
   string    - string.h (and legacy strings.h and non-standard nuttx/b2c.h)
@@ -155,5 +156,6 @@ Implementation Details
    :caption: Contents:
 
    search.rst
+   stdbit.rst
    stream.rst
    zoneinfo.rst

Documentation/components/libs/libc/stdbit.rst

@@ -0,0 +1,30 @@
+======
+stdbit.h
+========
+
+The optional C23 ``stdbit.h`` header provides bit manipulation macros
+(endianness, leading/trailing zeros and ones, count, single-bit test,
+bit width, bit floor, bit ceil). NuttX provides this header only when
+explicitly enabled via Kconfig.
+
+Configuration
+=============
+
+- **CONFIG_ARCH_HAVE_STDBIT_H** (bool, selected by arch)
+  Architecture indicates it provides ``arch/<arch>/include/stdbit.h``.
+
+- **CONFIG_ARCH_STDBIT_H** (bool "stdbit.h", depends on ARCH_HAVE_STDBIT_H)
+  Use the redirecting header. The build copies
+  ``include/nuttx/lib/stdbit.h`` to ``include/stdbit.h``; that header
+  then includes ``<arch/stdbit.h>`` when this option is set.
+
+- **CONFIG_LIBC_STDBIT_GENERIC** (bool "stdbit.h (generic C23)")
+  Use the generic C23 implementation. The same redirecting file
+  ``include/nuttx/lib/stdbit.h`` is copied to ``include/stdbit.h``,
+  and the generic implementation is used (no arch header). Requires
+  compiler builtins (e.g. ``__builtin_clz``, ``__builtin_ctz``,
+  ``__builtin_popcount``); see ``CONFIG_HAVE_BUILTIN_*`` in
+  ``nuttx/compiler.h``.
+
+Either **CONFIG_ARCH_STDBIT_H** or **CONFIG_LIBC_STDBIT_GENERIC** may be
+enabled so that ``#include <stdbit.h>`` is available.