Reorg doc directory

[mbkranz]

We may want to put docs into their respective folder (currently study-reg is in folder and everything else is in root doc directory). This was due to the inability to make a section homepage without a subdirectory -- this will prepare us for other section homepages in future and also increase consistency and readability

e.g., doc/platform/workspaces.md, doc/platofrm/discovery_page.md rather than platofrm_workspaces.md, platform_discovery.md etc