Fix dead links

## 1. https://amethyst.rs/doc
https://amethyst.rs/doc contains a dead link at
```html

Previous Stable Version (0.12)

```

Specifically, `https://docs.amethyst.rs/v0.12.0/amethyst/`returns a 404.

I don't know if they're still hosted under the amethyst.rs domain somewhere, alternatively you might want to link to `https://docs.rs/amethyst/0.12.0/amethyst/` instead.

I'm not sure how to open a PR to fix this, when I search for the strings I can't find them in this repo, at least not on the master branch. It's probably in a different part of your build / content pipeline.

## 2. [README.md](https://github.com/amethyst/website/blob/master/README.md)
README.md links to `https://amethyst-engine.org/` in line 3. The link appears to be unreachable.