[libc] Add a section about supported compilers in the documentation

[gchatelet]

Summary

Context https://github.com/llvm/llvm-project/issues/59368

libc/docs/compiler_support.rst

+.. _compiler_support:
+
+================
+Compiler Support
+================
+
+As of now only ``Clang`` is fully supported.
+
+We are in the process of supporting ``GCC`` but some memory function implementations rely on `compiler intrinsics`__ that are not currently available in ``GCC``.
+As such we cannot guarantee optimal performance for these functions.
+
+.. __: https://clang.llvm.org/docs/LanguageExtensions.html#guaranteed-inlined-copy
+
+Minimum version
+===============
+
+ - ``Clang 11``

libc/docs/index.rst

.. toctree::
    strings
    stdio
 
 .. toctree::
    :hidden:
    :maxdepth: 1
    :caption: Development
 
+   compiler_support
    build_and_test
    developer_guides
    porting
    contributing
 
 .. toctree::
    :hidden:
    :maxdepth: 1
    :caption: External Links
 
    Source Code <https://github.com/llvm/llvm-project/tree/main/libc>
    Bug Reports <https://github.com/llvm/llvm-project/labels/libc>
    Discourse <https://discourse.llvm.org/c/runtimes/libc>
    Discord Channel <https://discord.com/channels/636084430946959380/636732994891284500>
    Buildbot <https://lab.llvm.org/buildbot/#/builders?tags=libc>