[libc][docs] add sys/wait to documentation and include related functi… (

llvm#122598)

These changes ensure that the `sys/wait` header is documented properly
with respect to the issue (llvm#122006 ).

**Changes:**
1. **wait.yaml**: Created a new YAML file for `sys/wait` with functions
(`wait`, `waitid`, `waitpid`) and related macros.
2. **CMakeLists.txt**: Added `sys/wait` to the documentation
directories.
3. **index.rst**: Included `sys/wait` in the documentation index.

libc/docs/CMakeLists.txt

@@ -55,6 +55,7 @@ if (SPHINX_FOUND)
       strings
       sys/mman
       sys/resource
+      sys/wait
       threads
       uchar
       wchar

libc/docs/headers/index.rst

@@ -28,6 +28,7 @@ Implementation Status
    strings
    sys/mman
    sys/resource
+   sys/wait
    threads
    time
    uchar

libc/utils/docgen/sys/wait.yaml

@@ -0,0 +1,37 @@
+functions:
+  wait:
+    in-latest-posix: ''
+  waitid:
+    in-latest-posix: ''
+  waitpid:
+    in-latest-posix: ''
+
+macros:
+  WCONTINUED:
+    in-latest-posix: ''
+  WEXITED:
+    in-latest-posix: ''
+  WEXITSTATUS:
+    in-latest-posix: ''
+  WIFCONTINUED:
+    in-latest-posix: ''
+  WIFEXITED:
+    in-latest-posix: ''
+  WIFSIGNALED:
+    in-latest-posix: ''
+  WIFSTOPPED:
+    in-latest-posix: ''
+  WNOHANG:
+    in-latest-posix: ''
+  WNOWAIT:
+    in-latest-posix: ''
+  WSTOPPED:
+    in-latest-posix: ''
+  WSTOPSIG:
+    in-latest-posix: ''
+  WTERMSIG:
+    in-latest-posix: ''
+  WUNTRACED:
+    in-latest-posix: ''
+  WCORE_DUMPED:
+    in-latest-posix: ''