Documentation: Add Function Redirection API docs

Added a new page (functionredirection.rst) that describes the Function
Redirection (static stubbing) API. This page will be expanded if we add,
for example, ftrace-based stubbing.

In addition,
1. Updated the api/index.rst page to create an entry for function
   redirection api
2. Updated the toctree to be hidden, reducing redundancy on the
   generated page.

Signed-off-by: Sadiya Kazi <sadiyakazi@google.com>
Co-developed-by: Daniel Latypov <dlatypov@google.com>
Signed-off-by: Daniel Latypov <dlatypov@google.com>
Co-developed-by: David Gow <davidgow@google.com>
Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>