docs: clarify relative public_url usage

[leno23]

Summary

• Clarify that build.public_url can be a relative value such as ./.
• Add a trunk build --release --public-url='./' example for deployments that may be served from different prefixes, including GitLab Pages.
• Show the equivalent [build] configuration value.

Fixes #997

Test plan

• git diff --check
• Source-level docs check confirming the new --public-url='./', public_url = "./", and GitLab Pages guidance are present.
• mdbook test guide was not run locally because mdbook is not installed in this environment.