Add slug content to style guide

Add this info to the style guide (edit first):

> Best practices for slug writing (assuming you’ve already done keyword research):
>
> - Aim for 3-5 keywords in length.
> - Remove function words (articles, conjunctions) and any non-essential words as long as it makes sense.
> - When you translate a page, translate the slug in that language.
>

Find the repository's style guide and read its current slug guidance before editing. Add the supplied best-practices content, preserving the guide's existing format; done means the three slug-writing recommendations are present and the style guide remains consistent.

Written by the indexing model from the issue text.