Add copy of flat DAST docs to dast_flat

What does this MR do and why?

First in a series of MRs to reorganize the DAST documentation. This MR adds a copy of the flat DAST documentation to a dast_flat folder. Once merged the navigation links will be updated to use this folder. That will allow changes to the DAST documentation located in the dast folder without breaking navigation links.

This is needed because the navigation links are in a separate project.

Related to Reorganize documentation for DAST (#429537 - closed) • Michael Eddington • 16.9