New URL for the Bundled IO documentation page (#5767)

Summary:
Pull Request resolved: #5767

This diff is to rename the "sdk-bundled-io" documentation page to "bundled-io".

Old URL: https://pytorch.org/executorch/main/sdk-bundled-io.html
New URL ("sdk" is removed):
https://pytorch.org/executorch/main/bundled-io.html

Design doc: https://docs.google.com/document/d/1l6DYTq9Kq6VrPohruRFP-qScZDj01W_g4zlKyvqKGF4/edit?usp=sharing

Reviewed By: dbort

Differential Revision: D63651681

Author: Olivia-liu