Supermarket install docs are missing data_bag steps

[vinyar]

Location: https://docs.chef.io/install_supermarket.html

Specific step:

...
app = data_bag_item('apps', 'supermarket')
Set the attributes from the data bag:
...

Issue: Following the documentation will result in a data_bag not found failure.

Expectation: Document should include

• a functional .json example of the data_bag with placeholders for user-specific values
• should instruct the user to create the databag via knife data_bag create && knife data_bag from file ...
• link to data_bag documentation for comprehensive information
• note about using unencrypted data_bags for storing secrets.

[vinyar]

@kaoticfx