Add standalone getting started guides structure.

[avaidyanatha]

Thoughts

We have been designing the new format for the docs being displayed in the webapp for source and destination setup as per this issue: #3879 and its children. We thought we could just create a directory for each source and destination right where they lived, but this would massively change the directory structure, causing us to need to change things in a lot of different places. I propose a less invasive solution:

• Create another directory called getting-started and place all getting-started guides here initially. Have the naming convention be type-name e.g. source-facebook-marketing to be identified and loaded by the backend onto the image.

Main Changes

• Implements this directory structure and adds two guides (one for a source and one for a destination) for testing.