add md() processing of titles, subtitles, source notes

[alanahjonas95]

Wrap title, subtitle, and source_note values in gt::md() before passing them to tab_header() and tab_source_note()

Without md(), these elements currently render as plain text

resolves #611

[github-actions]

🦺 Coverage summary

🟢 Merging PR #664 (4c0e4a4) into main (6adafe4) will not change overall coverage.
🟢 Diff coverage is 100% (15 out of 15 modified lines are covered by tests).

♻️ Comment updated with the latest results.

^{Created on 2026-07-16 with covr2gh v0.0.0.9041.}