How-tos document to help user to understand how to use devfile in different scenarios

[elsony]

Add a how-tos section in documentation to help users to provide concrete examples to users on how to use devfiles (we can consider having separate how-tos section in each persona or group them together). For each how-to, it needs to provide examples of devfile with the explanation on each section of the devfile.

Some examples of how-to topics include:

1. Simple sample to complex example with steps/pointers/explanation
2. Adding a database
3. Adding a new component to my application

[rkratky]

JIRA: https://issues.redhat.com/browse/RHDEVDOCS-2869