Fix dead links

[nicmr]

1. https://amethyst.rs/doc

https://amethyst.rs/doc contains a dead link at

<h2>Previous Stable Version (0.12)</h2>
<!-- [... ] -->
    <li><a target="_blank" rel="noopener" href="https://docs.amethyst.rs/v0.12.0/amethyst/">Browse the API Reference</a></li>
<!-- [... ] -->
</ul>

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

README.md links to https://amethyst-engine.org/ in line 3. The link appears to be unreachable.

[nicmr]

Update for amethyst 0.15 website changes: Link is still dead, and now also outdated. (Trying to link to 0.12 but previous stable is 0.14).

<h2>Previous Stable Version (0.14)</h2>
<!-- [... ] -->
    <li><a target="_blank" rel="noopener" href="https://docs.amethyst.rs/v0.12.0/amethyst/">Browse the API Reference</a></li>
<!-- [... ] -->
</ul>]

In this case, there is no docs.rs alternative to link to, as docs.rs has been failing to build amethyst since 0.13. Is there a working docs.amethyst.rs link?