Release of The Memory Safety Continuum

[balteravishay]

This PR adds the Memory Safety Continuum document to the best.openssf.org website.

[david-a-wheeler]

@balteravishay - for now this seems like the right call.

In the longer term, do you want this document to look prettier, like the Concise Guide for Developing More Secure Software
? If you do, we need to implement some nice way to generate the web pages. We typically use the Simplest Possible Process (SPP), which uses GitHub Pages, unless you have a need for something fancier.