[libc][docs] Add sys/statvfs to documentation and YAML definitions (#126413)

These changes ensure that the sys/statvfs header is documented properly
with respect to the issue (
https://github.com/llvm/llvm-project/issues/122006 ) .

libc/docs/CMakeLists.txt

@@ -59,6 +59,7 @@ if (SPHINX_FOUND)
       sys/mman
       sys/resource
       sys/stat
+      sys/statvfs
       sys/time
       sys/wait
       termios

libc/docs/headers/index.rst

@@ -30,6 +30,7 @@ Implementation Status
    sys/mman
    sys/resource
    sys/stat
+   sys/statvfs
    sys/time
    sys/wait
    termios

libc/utils/docgen/sys/statvfs.yaml

@@ -0,0 +1,11 @@
+functions:
+  fstatvfs:
+    in-latest-posix: ''
+  statvfs:
+    in-latest-posix: ''
+
+macros:
+  ST_RDONLY:
+    in-latest-posix: ''
+  ST_NOSUID:
+    in-latest-posix: ''