Add a guide on making a bespoke `Titlebar` component

[dan-myles]

What kind of changes does this PR include?

• New or updated content
• Changes to the docs site code

Description

This PR adds a new guide that goes in-depth on making a custom titlebar for Tauri applications. It also goes over conditional rendering using some plugins. I think it'll be a great addition to the "learn" section, but can be moved around, of course.

I've also added a "bun" command to the command tabs component, needed it throughout the guide.

[netlify[bot]]

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[dan-myles]

Here is a link to the deployment preview and the specific article that was written: https://deploy-preview-2135--tauri-v2.netlify.app/learn/custom-titlebar/