Document Base self-demo script path

[codeforester]

Summary

• Document the Base self-demo script as $BASE_HOME/demo/demo.sh.
• Note that it follows the same --non-interactive pattern as project demo scripts.

Issue

Fixes #988

Validation

• git diff --check

Docs Impact

• Documentation-only self-demo reference update.

AI Context Impact

• None.

Demo Impact

• Clarifies the existing Base self-demo reference path; no demo behavior changed.