v2.25: Easier to find

Clearer page titles and descriptions, in the words Product Owners search for. No phrase repeats across pages.

• Home: Sprintwise: user story readiness checker for Product Owners.
• Product, Guide and Demo: each has its own title (the readiness check, checking stories before sprint planning, scoring a sample backlog).
• New FAQ answer: what a Definition of Ready is, and how Sprintwise checks it.
• About page: says "code you can read on GitHub" instead of "open source", which matches the v2.24 license.