docs: fix "docs introduction" link in Fast Website & github page

[ChrisRoald]

Hi, this link - https://fast.design/docs/introduction/
fails to open in both the Fast website and the Github page.

[janechu]

@ChrisRoald where is the link coming from? We have done a docs update that versions everything so that would be why it is now broken.

Edit: Oops, I see it's in the README.md, will fix, thanks!