chore: remove unused CSS and fonts, add fonts we use

[johncowen]

Following on from https://github.com/kumahq/kuma-gui/pull/3689 I had another quick review of our global CSS and found a few more things we could remove there.

Interestingly I also found out (from what I could see) that we ship the Fira Mono but we don't seem to use it anywhere? Whereas @kong/design-tokens tokens use JetBrainsMono (kui-font-family-code) but we don't ship that even though we say some of our fonts should use it if its available (it has fallbacks, but still). Honestly I find it a little weird that @kong/design-tokens doesn't ship the fonts it uses, even if they wanted that to be optional, but 🤷

I removed Fira Mono and added JetBrainsMono from here https://github.com/JetBrains/JetBrainsMono

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[johncowen]

💯 there should be tokens for typography

btw there are tokens for typography, but they don't ship the fonts that they define in the tokens 🤷 , even so you could pull them in optionally (or use google fonts)