docs: how-to for porting plugins from FPF to frontend-base

Adds a site-ops how-to that walks plugin authors through porting both
halves (npm package + Tutor plugin) of an existing frontend-plugin-framework
contribution to frontend-base, with translation tables for imports, slot
ops, slot ids, and tutor-mfe patches. Cross-linked from the existing
use-frontend-plugin-slots page.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: arbrandes