Add preview workflow

[justinjao]

What?

Fix #1908

This PR constitutes the work done for the Google Summer of Code project for Justin Jao, as referenced here.

This adds a proposed workflow for automating the deployment of a staging environment upon PR creation to the datahub repo.

In particular:

• a preview.yml workflow file that triggers the automated deployment of the staging environment with the new studies imported
• a preview_closed.yml file that automatically tears down the deployed staging environment upon PR closure or merging.
• the directory and miscellaneous file infrastructure (located within the added preview directory) necessary to support this automated deployment
• documentation for this proposed workflow, located within the documentation directory

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Important Notes:

• This PR currently must undergo further discussion to determine which account will setup the Okteto infrastructure needed for this workflow to function
• After this has been determined, the namespace listed in the code will need to be changed accordingly within each of these files
• Before merging, certain steps, such as adding Okteto secrets, must be undertaken (documented in Preview_Setup.md) before the workflow will be fully functional
• Currently, not all studies may be imported, as there is a known issue with gene panels IDs not being present in the seeded database. This may be fixed in a future update, but for now, a temporary stopgap has been placed (documented in Preview_Overview.md).
• For more information about this workflow, please see the produced documentation at Preview_Overview.md.