Create Overview Document

[hai1337]

Create the document, /overviewandarchitecture/overview.md, describing the overview of OSC. Currently, the landing page points to an overview in the architecture document. Once the new Overview document is created, ensure /overviewandarchitecture/overview.md points to the document.

This content should not be the same as the generic introduction in the architecture document; it should be specific to OSC.

Ensure these changes are cleanly integrated into the osc.org website.