Various documentation improvements for 2.1

- Remove all references to slick-examples and point to the Activator
  templates instead.

- To reduce the maintenance burden, external samples are not tied to the
  minor version or qualifier anymore. We now have 4 official Activator
  templates plus slick-testkit-example (still a regular sbt project
  because testing and error reporting does not work properly in
  Activator with our custom TestKit tests) which are all linked from the
  manual. We now link to the major release branches which can be
  updated independently when we release new minor versions. Activator
  templates have to be published separately for different major versions
  (because Activator does not support versioning yet) but multiple
  versions can live in different branches of the same repository.

- Populate the previously empty Index page.

- Add a Compatibility Policy section.

- Move the search box to the top (where people expect it) and remove the
  accompanying text (which refers to Python-specific concepts).

- Change “documentation” to “manual” in the title (short enough to
  prevent overflow with SNAPSHOT versions).

Fixes #603. Review by @cvogt