Expose page aliases to templates

[chatman-media]

Summary

• serialize page aliases so templates can access page.aliases
• document the new Page variable
• add unit and integration coverage

Closes #3167

Checks

• cargo fmt --all --check
• cargo test -p content -p zola page_serialization_includes_aliases
• cargo test -p site -p zola can_build_site_without_live_reload