Improve SEO for Infinispan documentation.

Part one: Drop old documentation: https://infinispan.org/docs/stable/titles/overview/overview.html
Content in the Overview is either redundant or out of date.

Part two: Make sure the canonical URLs for titles docinfo.html points to the correct stable page for that title, instead of the index.

Part three: Improve titles with sentence style capitalization, add meaningful abstracts to each title, and update the index for three content types, getting started and contributing, cluster operations, developing with Infinispan cache capabilities.