New URL for the Bundled IO documentation page

Summary:
This diff is to rename the "sdk-etrecord" documentation page to "etrecord".

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

Differential Revision: D63651681

Author: Olivia-liu