docs/style.md: Make the page self-contained

This page assumes the reader is coming from another page (presumably the "Styling" section of the Introduction page at docs/index.md), so it refers to "the" MapLibre styling and does not link to the style spec.

It's usually better to make documents able to stand alone, so that people can use them effectively regardless of how they arrived at them. This MR slightly adjusts the opening sentence of this file to help in that regard.