Release Notes & Webpage

Hi,

so, I was wrong, `llvm-project/openmp/docs/ReleaseNotes.rst` exists already.
Please populate this file *asap* as it is about LLVM 11.

I also took a look at `http://openmp.llvm.org` again, which lives in `llvm-project/openmp/www/index.html`
and it looks really outdated and pretty sparse. As a first step, I would actually like to change the design.
With that I want to add some content that is up to date, and the placeholders for new functionality, e.g.,
the error explanation section, FAQ, command line options, ...

I can certainly not do this alone so once we start (= next week?) I would hope people will chime in and provide content.

~ Johannes