Update API reference
Remove note about action callable limitations

Action specs in route definitions accept all callable types as of
398b5a769f50df8544937c8a60fe00021e3de4e2.
Add note about null actions to routes overview
Use more descriptive prompts in shell examples

For shell examples in the Creating Your First HotMelt Site tutorial,
include the assumed working directory in the prompt.
Include default config.php in first site tutorial
Add configuration overview
Add references to configuration overview

Remove info about host name-based configuration from the Creating Your
First HotMelt Site tutorial and point to new configuration overview
instead.

Change host named-based configuration info link to configuration
overview (instead of HotMelt\Config class reference).