docs: add optional Backer Concierge AI Agent lesson to cli harness

Add lesson 8 (optional) walking learners through building a
catalog-grounded Backer Concierge hosted agent with GitHub Copilot
CLI and the Microsoft Foundry Skill, and renumber the review lesson
from 8 to 9 to make room for it.

- Wire the new lesson into the sidebar, harness README, lesson 7's
  closing navigation, and the review lesson, following the
  '<n>. <title> (Optional)' label convention and offering an explicit
  skip-ahead link for learners who aren't interested.
- Fix translated (es-es, ja-jp, ko-kr, pt-br, zh-cn) README and lesson
  7 next-lesson links that pointed at the now-renamed 8-review.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: juliamuiruri4