Point Liquid links to Liquid’s Github wiki

I did not change the number of links; even though they're all pointing to the same page, they point to specific anchors within that page.

Are there any other Liquid links that should be updated?
 - [x] [`site/_docs/variables.md`](https://github.com/jekyll/jekyll/blob/d950680bbe48bebad615c61bda2502dbd2feba2b/site/_docs/variables.md#L12)
 - [x] [`site/_docs/templates.md`](https://github.com/jekyll/jekyll/blob/6644c77d234a8bf9e6e8ed8745389ee20be21afd/site/_docs/templates.md#L10)
 - [x] [`site/_docs/index.md`](https://github.com/jekyll/jekyll/blob/3e98860241553febaeef10cb44304cbe0903f4bc/site/_docs/index.md#L20)
 - [x] [`site/index.html`](https://github.com/jekyll/jekyll/blob/c8b22a19adee0d2dfe3418349b314c35dd8c1212/site/index.html#L25)

:question: 

Hopefully resolves questions like #2883