SPA (Single-Page-Application) - Performance Optimizations

[ashermyers]

Summary

This adds the Astro-SPA plugin which improves performance immensely by using client side rendering.

Screenshots (optional)

Documentation checklist

• [ X ] The documentation style guide has been adhered to.
• [ X ] If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• [ X ] Files which have changed name or location have been allocated redirects.

[Cherry]

In my very brief testing, this unfortunately seems to make things generally worse, with tonnes of duplicate prefetches, not to mention astro-spa seems to be quite unmaintained, with ViewTransition being a more ergonomic and modern solution.

It might make more sense to encourage Starlight to build this natively into the framework at https://github.com/withastro/starlight and then use that when it's ready?

[kodster28]

Based on these thoughts from @Cherry + our own investigations into ViewTransitions (which also isn't quite the way to go atm), going to close this out.

Thanks for the contribution though! If you have other ideas like this in the future, more than open to re-evaluating them then.