Fix links in documentation.

Some method links in connection.rst still don't light up in scaladoc
(but at least the pages are now correct). They are probably wrong but
guessing the right ones is a nightmare. Scaladoc needs to add a way to
get proper links out of the generated pages to make this usable.
Update documentation for Lifted Embedding

- Better explanation for lifting
- New "Compiled Queries" feature
- MappedTo for automatic mapping of column types
- Fix links to API docs
- Fix overflow scrolling in new docs theme
Add migration guide 1.0 -> 2.0
Improved Lifted Embedding documentation.

- Use matching examples for plain and Rep types
- Show typical SQL statements
- Remove "soft insert" code snippet and paragraph
- Some small fixes
Refactoring of the user manual.

- Lifted Embedding chapter split up into Schemas, Queries and
  User-Defined Features.
- Overview of the query APIs in the Introduction chapter.
- Move Slick Extensions chapter further up, Direct Embedding
  and Slick Testkit down to the bottom.
- Clearly mark Direct Embedding as experimental.
- Put the Migration Guide near the top.
Merge pull request #547 from slick/tmp/doc-improvements-rc1

Some doc improvements for RC1