v3 docs availability and migration guide

[benmccann]

Describe the problem

When we launch the v4 docs, we should make sure they cover v3 as well. I don't think we need a separate version of the docs for v3 since the changes aren't that large, but can just call the differences out inline

Describe the proposed solution

I think we could note these changes inline:

• |local default for transitions in v4
• format option being available for v3 (chore: unbundled esm #8613)
• note that a11y no-noninteractive-element-interactions and a11y-no-static-element-interactions only available in v4
• note that `{#each}1 only works on iterables starting in v4

The custom elements changes are larger, but still possibly could be handled inline. We could also say if you're still on v3, see the migration guide for a comparison of the APIs. Or maybe we make a /docs/custom-elements-v3 page that's only linked to from the v4 custom elements docs and leave it out of the menu if we really need more docs.

Some of the types are also different, but I think we can just put a note at the top to say a few of the types have changed. If you're on v3, please use your IDE to reference the types.

Also of note, we need to make sure the REPL works in v4: sveltejs/sites#486

Alternatives considered

Whole separate site for v3, host multiple versions with /docs with a version switcher (we'll probably want to do this when v5 comes so maybe do it early?), or throw users to the wild

Importance

nice to have

[dummdidumm]

• local vs global is already noted inline
• custom element tag thing should be noted, that's still missing
• I don't think we need any note on the a11y warnings. If you're on 3.20 for example you'll also don't have some of the newer a11y warnings, and we don't note that, too
• iterables should be noted
• format option should be noted

[benmccann]

The docs for Svelte 3 / Svelte 4 can be found at https://v4.svelte.dev/