docs: Add stdbit.h to libc index and legacy README.

Add stdbit.h to the libc index and legacy README so that the
new stdbit.h header is discoverable in the documentation and
listed alongside other libc headers.

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

Author: arjav1528

Documentation/components/libs/libc/index.rst

@@ -52,6 +52,7 @@ functions.  So we have::
   search    - search.h
   semaphore - semaphore.h
   stdio     - stdio.h
+  stdbit    - stdbit.h (C23, optional; see CONFIG_ARCH_STDBIT_H / CONFIG_LIBC_STDBIT_GENERIC)
   stdlib    - stdlib.h
   string    - string.h (and legacy strings.h and non-standard nuttx/b2c.h)
   time      - time.h

Documentation/legacy_README.md

@@ -819,6 +819,25 @@ you can use an architecture-specific `stdarg.h` header file by defining
 See the discussion above for the `math.h` header.  This setting works
 exactly the same for the `stdarg.h` header file.
 
+### stdbit.h (C23)
+
+The C23 ``stdbit.h`` header provides bit-manipulation macros (leading/trailing
+zeros and ones, count, bit width, bit floor/ceil, endianness, etc.).  NuttX
+keeps a redirecting header at ``include/nuttx/lib/stdbit.h``.
+
+* **CONFIG_ARCH_STDBIT_H**: Enable when the architecture provides
+  ``arch/<architecture>/include/stdbit.h``.  The architecture must select
+  ``ARCH_HAVE_STDBIT_H`` in its Kconfig.  The build then copies
+  ``include/nuttx/lib/stdbit.h`` to ``include/stdbit.h``, which includes the
+  architecture-specific header.
+* **CONFIG_LIBC_STDBIT_GENERIC**: Enable to use the generic C23 implementation
+  on any architecture (no arch-specific header required).  The same redirecting
+  header is copied to ``include/stdbit.h`` and the in-header generic fallback
+  is used.
+
+Use either option when your toolchain does not provide ``stdbit.h`` and you
+need C23 bit utilities.
+
 # CONFIGURING NUTTX
 
 ## Instantiating "Canned" Configurations