Persistent `iframe` or alternative

[rusty-key]

Currently, reshowcase is reloading iframe every time we go from demo to demo. This causes fetching/parsing/executing all assets:
https://github.com/ahrefs/reshowcase/assets/1197026/e6705a68-2cd9-4c3a-8ad6-4122e77e38e8

It makes reshowcase navigation unnecessarily slow, especially on large applications.

Instead, we can make iframe persistent, and only replace its content. This way, reshowcase will fetch assets only once and only need to render relevant demo on navigation

We want to find a solution to navigate between stories without full reload of the application