[libc][docs] split up platform and arch support (#120125)

Creates a new toctree "Support" under which we have distinct links to arch,
platform, and compiler support.

* Moved "Platform Support" from index landing page to new doc.
* Created explicit "Architecture Support". Requested in #118964 (comment).
* Moved "Compiler Support" from Status toctree to new Support toctree.

---------

Co-authored-by: Carlo Cabrera <github@carlo.cab>

libc/docs/arch_support.rst

@@ -0,0 +1,19 @@
+Architecture Support
+====================
+
+The currently continuously tested architectures are:
+
+* aarch64
+* amdgpu
+* arm
+* nvptx
+* riscv32
+* riscv64
+* x86_64
+
+i386 support is [in the works](https://github.com/llvm/llvm-project/issues/93709).
+
+See "`Bringup on a New OS or Architecture <porting.html>`__" for more
+information. Please do first file a bug in
+`our issue tracker <https://github.com/llvm/llvm-project/labels/libc>`__ before
+starting a port that you plan to upstream.

libc/docs/index.rst

@@ -35,24 +35,6 @@ LLVM-libc aspires to a unique place in the software ecosystem.  The goals are:
   algorithms.
 - `Fuzzing <https://github.com/llvm/llvm-project/tree/main/libc/fuzzing>`__
 
-Platform Support
-================
-
-Most development is currently targeting Linux on x86_64, aarch64, arm, and
-RISC-V. Embedded/baremetal targets are supported on arm and RISC-V, and Windows
-and MacOS have limited support (may be broken).  The Fuchsia platform is
-slowly replacing functions from its bundled libc with functions from this
-project.
-
-LLVM-libc does not guarantee backward compatibility with operating systems that have reached their EOL.
-Compatibility patches for obsolete operating systems will not be accepted.
-
-For Linux, we support kernel versions as listed on `kernel.org <https://kernel.org/>`_, including
-``longterm`` (not past EOL date), ``stable``, and ``mainline`` versions. We actively adopt new features
-from ``linux-next``.
-
-For Windows, we plan to support products within their lifecycle. Please refer to 
-`Search Product and Services Lifecycle Information <https://learn.microsoft.com/en-us/lifecycle/products/?products=windows>`_ for more information.
 
 .. toctree::
    :hidden:
@@ -70,10 +52,18 @@ For Windows, we plan to support products within their lifecycle. Please refer to
    :maxdepth: 1
    :caption: Status
 
-   compiler_support
    headers/index.rst
    c23
 
+.. toctree::
+   :hidden:
+   :maxdepth: 1
+   :caption: Support
+
+   arch_support
+   platform_support
+   compiler_support
+
 .. toctree::
    :hidden:
    :maxdepth: 1

libc/docs/platform_support.rst

@@ -0,0 +1,22 @@
+Platform Support
+================
+
+Development is currently mostly focused on Linux.  MacOS and Windows has
+partial support, but has bitrot and isn't being tested continuously.
+
+LLVM-libc is currently being integrated into Android and Fuchsia operating
+systems via `overlay mode <overlay_mode.html>`__.
+
+For Linux, we support kernel versions as listed on
+`kernel.org <https://kernel.org/>`_, including ``longterm`` (not past EOL
+date), ``stable``, and ``mainline`` versions. We actively adopt new features
+from ``linux-next``.
+
+For Windows, we plan to support products within their lifecycle. Please refer to 
+`Search Product and Services Lifecycle Information <https://learn.microsoft.com/en-us/lifecycle/products/?products=windows>`_ for more information.
+
+LLVM-libc does not guarantee backward compatibility with operating systems that
+have reached their EOL. Compatibility patches for obsolete operating systems
+will not be accepted.
+
+For GPU, reference `our GPU docs <gpu/index.html>`__.