Add a link to the QuTiP Virtual Lab.

[hodgestar]

An initial pass at adding QuTiP Virtual Lab to the website.

The plan is to use the blurb here as a page to link to when announcing QVL elsewhere.

Things left todo:

• [ ] @trentfridey @ajgpitch reviews of the blurb text would be particularly welcome.
• [ ] Check whether this even renders in whatever version of Bootstrap is used by the renderer.
• [ ] Add a news item.

Probably the QuTiP website needs a bigger revamp, but I'm declaring that out of scope of this PR. :)

[hodgestar]

Thanks for reviewing @trentfridey. Sorry this took a long time to get back to.