[libc] Website fixes (sidebar and mobile)

[jeffbailey]

Summary

Add "using" and "status" sections to the sidebar to make getting these
easier.

Fixed mobile formatting not overflow left and right.

Tested:
Chrome on Desktop, using mobile restrictions in devtools.

libc/docs/_static/custom.css

 pre {
   padding: 7px;
 }
 
 div.document {
   width: 90%;
 }
 
-div.body {
-  max-width: 90%;
-}
-
 div.note {
   padding: 20px 20px 10px 20px;
 }
 
 div.note p.admonition-title {
   font-size: 130%;
 }
 

 div.sphinxsidebar ul li.toctree-l1 > a {
   font-size: 100%;
 }
 
 div.sphinxsidebar h3 {
   /* Hide Navigation. */
   display: none;
 }
+
+@media screen and (min-width: 875px) {
+div.body {
+  max-width: 90%;
+}
+}
+
+@media screen and (max-width: 875px) {
+div.sphinxsidebar {
+  margin-left: 0px;
+  margin-right: 0px;
+}
+
+div.body {
+  min-width: auto;
+  max-width: auto;
+}
+}

libc/docs/index.rst

 
 The libc is written to be ABI independent.  Interfaces are generated using
 LLVM's tablegen, so supporting arbitrary ABIs is possible.  In it's initial
 stages there is no ABI stability in any form.
 
 .. toctree::
    :hidden:
    :maxdepth: 1
-   :caption: Documentation
+   :caption: Using
+
+   runtimes_build
+
+.. toctree::
+   :hidden:
+   :maxdepth: 1
+   :caption: Status
+
+   strings
+   stdio
+   math
+
+.. toctree::
+   :hidden:
+   :maxdepth: 1
+   :caption: Development
 
    build_system
    clang_tidy_checks
    entrypoints
    fuzzing
    ground_truth_specification
    header_generation
    implementation_standard
    api_test
    layering
    mechanics_of_public_api
    redirectors
    source_layout
-   strings
-   runtimes_build
-   stdio
-   math
 
 .. 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/issues/labels/libc/>
+   Bug Reports <https://github.com/llvm/llvm-project/labels/libc>