Improve CLI logging messages (e.g. for incremental builds)

We should improve the CLI logging messages e.g. in the following cases:

- timing: we should add how long a step has taken (e.g. 50ms to generate 20 documents)
- This should also provide a summary of how time is spent in different remark plugins etc
- Adjust log messages for incremental builds based on file watching. (Currently the log messages indicate that all documents have been re-generated even though just a single file has changed)
- The log messages should also indicate when using cache from file system

Ideally we can even easily test the wanted behaviour above in unit tests.