DOCS: Make it more clear what is svelte 4 and svelte 5 on the website

[Falk33n]

Describe the problem

Many people look at the docs and believe that what they are doing is svelte 5 but in reality they have read svelte 4 documentation

Describe the proposed solution

Add more eye catching warning that some code is svelte 5 and some is svelte 4

Importance

i cannot use svelte without it

[kwangure]

Kindly include links to the specific docs referring to Svelte 4 features legacy APIs that give the impression they're supported in future Svelte 5 versions.

It's safe to assume that anything under the "Legacy APIs" section is Svelte 4 and will be dropped in future versions. Anything under "Runes" is Svelte 5 onwards only. Anything else likely existed in Svelte 4 and will continue to be supported in Svelte 5 and future versions.

[dummdidumm]

Can you give examples for where it's unclear? Everything under the legacy section is legacy, and everything else is not. I'm not sure what else you need