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 5 official Activator
  templates which are all linked from the manual. We 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.
Merge pull request #811 from slick/tmp/activator-doc-changes

Various documentation improvements for 2.1
Use Typesafe Config for database configurations

- JdbcBackend gets a new factory method Database.forConfig for loading
  a configuration from application.conf or a custom config object. There
  is no built-in pooling support yet but the basic configuration keys
  are compatible with Play’s database configuration.

- We also use Typesafe Config for TestKit so we don’t have to “parse”
  a custom properties file and perform substitutions on it anymore. All
  defaults and substitutions are now in testkit-reference.conf.

- The main TestKit configuration is loaded from test-dbs/testkit.conf
  which is merged with the reference configuration so you don’t have to
  copy any default values into your own config file. This makes the
  configuration more resilient to changes in the defaults.
More flexibility for TestKit

- Move DBTest and the standard TestDBs into TestKit to allow other
  projects to be easily tested against the standard databases.

- Move the list of TestkitTests to run into testkit-reference.conf