Rebrand: GitHub Pages docs site header #1030

Part of #1024. Step 6, review the design first.

• docs/_config.yml: title and description drive the site header, the browser tab title, SEO tags and search.
• docs/_includes/title.html: replace the ' for VS Code' non-breaking-space workaround. Decide the header layout for the longer name (for example "DocumentDB" plus a smaller subtitle).
• Check wrapping on desktop and mobile, light and dark.