Add a "Settings" button to the dashboard which provides helpful information

[opcode81]

The button shall not really allow to change any settings but provide information on where/how to configure things, i.e. as a point of documentation for users. In particular, it shall point to

• serena_settings.yml
• project.yml
• command-line options.

As some users wonder how to disable the dashboard appearing by default, we can mention that this can be done in serena_settings.yml.