Descriptive title element and blurb for instruction pages

In #261 @jsha writes: We should also ensure each page has a title that reflects what the page is about, e.g. "How to get a Let's Encrypt certificate on Ubuntu with Certbot and Apache," and a 1-sentence blurb at the top to give context to people arriving directly from a search rather than from the main page.

Creating a separate issue for this - I think it's a good idea.