fix: date sitemap entries from their pages

The sitemap gave every page the build date. A page now states its own date
and a mkdocs hook passes it on, or leaves lastmod out. The values added to
the site's code spans today are also escaped: a backtick in a profile or
file name could close the span and inject markup.

scripts/mkdocs_hooks.py

@@ -0,0 +1,17 @@
+"""MkDocs hooks for the generated site.
+
+The sitemap gave all 664 pages the build date as lastmod, so the field told
+a crawler nothing. A page whose content carries its own date (an emulator
+page: the day its profile was last read against the source) states it in
+its front matter; the rest get no lastmod rather than an inexact one.
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+
+def on_page_markdown(markdown: str, page: Any, config: Any, files: Any) -> str:  # noqa: ARG001
+    """Take lastmod from the page's own date, or leave it out."""
+    page.update_date = str(page.meta.get("date_modified") or "")
+    return markdown