fix: preserve angle brackets in code when exporting Markdown

[mishig25]

Summary

• The View as Markdown button (and the .md companion files) were silently dropping <...> content. Two failure modes:
  • Placeholders eaten: literal placeholders inside fenced code (e.g. <YOUR_SUBSCRIPTION_ID>) were stripped because strip_remaining_html matched them as HTML tags.
  • Snippets nuked: a stray < in code (e.g. if idx < n:) plus any > later in the page (e.g. mask_img > 0) caused the regex <[^>]+> to eat everything between them across line boundaries — wiping entire code blocks and surrounding prose. (Reported by @juanju and @Mishig.)
• Fix in src/doc_builder/utils.py::strip_remaining_html:
  • Stash fenced (``` ... ```) and inline (`...`) code into placeholders before any HTML stripping; restore them afterward, so brackets inside code survive verbatim.
  • Tightened the generic-tag fallback from <[^>]+> to <[a-zA-Z/!][^>\n]*>: requires a tag-like character right after < and forbids matches across newlines.
• Added regression tests in tests/test_utils.py covering both screenshots' scenarios plus a positive test that real tags (<Tip>, <div>) are still stripped.

Test plan

• uv run pytest tests/test_utils.py -v — all 5 tests pass (3 new + 2 existing)
• uv run ruff check src/doc_builder/utils.py tests/test_utils.py — clean
• uv run ruff format src/doc_builder/utils.py tests/test_utils.py — clean
• Manually verify on the affected pages (Microsoft Foundry / Meta SAM 3 docs) once deployed

🤖 Generated with Claude Code

cc @alvarobartt