refactor: convert index.md and brag.md to maximum Markdown

- Add markdown="1" to all text-bearing divs (bento-card__body,
  project-card__main, talk-card__body, mentoring-card, section__header)
- Replace <strong> with **bold** Markdown syntax throughout
- Replace &amp; with literal & in Markdown-processed text
- Replace <img> with Markdown image syntax + IAL {: .hero__avatar}
- Replace <h1>/<h2> with # / ## headings + IAL classes and ids
- Convert inline links to [text](url) Markdown syntax in descriptions
- Use kramdown IAL {: .classname} to apply CSS classes to Markdown elements
- Enrich content from GitHub profile (copilot-instructions repo, MCP.Pack detail)
- Remove verbose HTML comments in favour of brief inline comments

Agent-Logs-Url: https://github.com/SebastienDegodez/SebastienDegodez.github.io/sessions/2df3b8ec-db7f-42f7-8400-22786ae23691

Co-authored-by: SebastienDegodez <2349146+SebastienDegodez@users.noreply.github.com>

Author: Copilot