Remove references to `hydratable` compiler option in docs

[bluwy]

Describe the problem

Svelte 5 had removed the option as components are always hydratable by default now. However the docs, and a couple places in code, still referenced it.

Describe the proposed solution

Remove references to hydratable compiler option in docs (and some code)

Alternatives considered

Not breaking anything and can be ignored, but could be misleading.

Importance

nice to have