docs: document SEO title/description templating and its scope

Documents the new title/description {date} templating mechanism, the
short-date format, and that webTitle is no longer used for
<title>/og:title/twitter:title (kept only for the share button).
Notes the target-search-terms-are-reference-only decision, confirms
crawl/index behaviour already matches product requirements for all 6
V0 puzzle pages (doNotIndex() would never trigger for them in prod),
and documents which other pages from the same SEO spreadsheet (hub,
group landing pages, archive pages) are out of scope because they
don't exist in this codebase yet.

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

Author: andresilva-guardian