Update navbar links with new URLs from restructured docs

[neiljdo]

The navbar links need to be updated with the new URLs following the docs restructuring. This will be possible after some updates to the quantinuum-ui and quantinuum-sphinx repos by @irfankhan10 that allow for customization by product owners.

NOTE: All URLs are prefixed by the product name, e.g. https://docs.quantinuum.com/lambeq/ + page/url

• 'Getting Started': as-is
• 'User Guide': pipeline.html → guide/pipeline.html
• 'Tutorials': as-is
• 'Code Examples': as-is
• 'API Reference': remove this