[libc][docs] add redirect for math/index.html

commit a9aff44 ("[libc][docs] reorganize documentation (llvm#118836)")

moved https://libc.llvm.org/math/index.html to
https://libc.llvm.org/headers/math/index.html which makes links from various
slide decks stale.

There's an extension for sphinx that can generate redirects.  Add a dependency
on that, then use it to create a redirect so that those older links still work.

I was able to install this sphinx extension via:

    $ sudo apt install python3-sphinx-reredirects

We may need to install this on whatever server generates the llvm
documentation.

libc/docs/conf.py

@@ -25,7 +25,11 @@
 
 # Add any Sphinx extension module names here, as strings. They can be extensions
 # coming with Sphinx (named 'sphinx.ext.*') or your custom ones.
-extensions = ["sphinx.ext.intersphinx", "sphinx.ext.todo"]
+extensions = [
+    "sphinx.ext.intersphinx",
+    "sphinx.ext.todo",
+    "sphinx_reredirects"
+]
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ["_templates"]
@@ -261,3 +265,7 @@
 
 # Enable this if you want TODOs to show up in the generated documentation.
 todo_include_todos = True
+
+redirects = {
+  "math/index": "../headers/math/index.html"
+}