* docs: bring doc titles and descriptions into SEO length limits
Rewrite the frontmatter title and description of every published doc so the
rendered <title> lands in 50-60 characters (including the site suffix) and the
meta description lands in 150-160 characters.
* docs: move SEO titles to a seoTitle field and restore display titles
The frontmatter title renders as the page H1 and the sidebar label, so tuning it
for the title tag shortened nothing and lengthened the visible navigation. Put
the search-facing string in seoTitle and give title back its original value.
Julian Sommer