Integrate new OpenCilk README.md with documentation

[behoppe]

The new OpenCilk README.md has a lot of good material for the website (see OpenCilk/opencilk-project#95). From that content, we may want to create new website pages for topics like cilk_for and Cilksan; and we may want to think about how to manage content for README topics like "An introduction to Cilk programming" that also have their own existing documentation.

[behoppe]

@neboat how would you like to approach documentation as we anticipate changes to cilk_for from OpenCilk/opencilk-project#303? There is a dormant PR for documenting cilk_for here, and more recent cilk_for documentation in the OpenCilk README.md, and an open question here about where we want to publish documentation, so that people don't get confused looking at the wrong place. How can I help us maintain our cilk_for documentation? I personally agree with @wheatman that www.OpenCilk.org would be a good source of "documentation truth" for all other places to point to, but I will support whatever approach you are behind.