docs: Add documentation for the io.IOBase class.

This provides a reference for users creating custom stream-like objects.

Co-Authored-By: Gemimi Pro 2.5 <gemini-2.5-pro-exp-03-25@google.com>
Signed-off-by: Andrew Leech <andrew.leech@planetinnovation.com.au>

Author: pi-anl

docs/library/index.rst

@@ -68,6 +68,7 @@ library.
    hashlib.rst
    heapq.rst
    io.rst
+   io.IOBase.rst
    json.rst
    marshal.rst
    math.rst