* Add index preprocessor
README.md is a de facto index file in markdown-based documentation.
Hence, we respect to README.md and convert it into index.html.
* Fix warning for unused variables
* Update tests for config
* Match file stem case-insensitively for IndexPreprocessor
* Add tests for IndexPreprocessor
* Update book example to fit index preprocessor
As `theme` dir is no longer under `src`. Updates to "theme" did not
trigger book rebuild.
Also fixed misleading docs about `theme` dir being located in `src`
