[libc][docs] disable pthreads docs

Having a target named pthreads is breaking when multiple runtimes are enabled.
Disable this target for now so that the builds go back to green (and sites get
updated).

Link: llvm/llvm-zorg#359 (comment)
Link: #122006
Link: #122497
Link: #123821

Author: nickdesaulniers

libc/docs/CMakeLists.txt

@@ -45,7 +45,8 @@ if (SPHINX_FOUND)
       locale
       net/if
       netinet/in
-      pthread
+      # TODO: https://github.com/llvm/llvm-project/issues/123821
+      # pthread
       setjmp
       signal
       stdbit

libc/docs/headers/index.rst

@@ -17,7 +17,6 @@ Implementation Status
    math/index.rst
    net/if
    netinet/in
-   pthread
    search
    setjmp
    signal
@@ -37,3 +36,6 @@ Implementation Status
    uchar
    wchar
    wctype
+..
+   TODO: https://github.com/llvm/llvm-project/issues/123821
+   pthread