Update fonts-setup.md with SASS url() tip

[talss89]

Users aren't immediately aware of the pitfalls of using url() imports in SASS when reading the Sage font setup documentation.

• Added tip to 'Add the CSS' section, highlighting the difference between url() imports in SASS vs CSS.
• Also referred to the docs for bud.assets() to copy fonts into dist.

See user story from Discourse thread: https://discourse.roots.io/t/cant-embed-woff2-font-in-sage10/25413/5

[talss89]

Unsure if my wording is concise enough, but it's a start.

[retlehs]

Thank you for the PR! I'm hesitant to add any Sass specific information to this guide — would rather just recommend using the relative ../fonts/ directory instead 🤔

I'm thinking about splitting up the Replacing Tailwind with Bootstrap guide into two separate guides:

1. Adding Sass to Sage (which would cover this specifically)
2. Replacing Tailwind with Bootstrap

I'll make sure you're added as an author via both git and in the docs credits in the new guide

[talss89]

Splitting and adding the detail to the 'Replacing Tailwind' guide makes much more sense! It did feel like a squeeze as I was writing this tip.

Just wanted to make sure it was captured, as I feel like I've seen this crop up a few times now. Will close this PR.

Thanks!