Document FPF examples (which ones work with compat vs not)

[brian-smith-tcril]

For each example we have in the FPF slot READMEs:

• Is it ✅ Working?
• Is it ❗ Not supported? (with link to docs for how to handle that use case now)
• Is it ❌ Not supported, no replacement.

Should check all the examples from FPF slots in:

• frontend-component-header
• frontend-component-footer
• frontend-app-authn
• frontend-app-learner-dashboard