(feat) Adding styling to code blocks

[userbradley]

I noticed that the code blocks lack styling, which makes them just a little bit harder to read.

This PR updates the mkdocs.yaml file to add in the required styling as per the documentation

As far as I know, there should not be any additional imports required for the mkdocs GHA

Before

After

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note: I have got a previous CLA with Google

[imjasonh]

Thank you! This looks much better

[userbradley]

Always a pleasure. RE my beginner mistake (No new line) - Would you like a pre-commit file to ensure it's included?

This is the pretty standard one I use: https://github.com/userbradley/documentation.breadnet.co.uk/blob/main/.pre-commit-config.yaml