docs: fix docs on mutating props

[Rich-Harris]

The docs added in #13994 are helpful but also somewhat misleading: the message should be a loud and clear 'don't mutate props' followed by details and examples, rather than an illustration of mutating a prop that does work but without mentioning the warning that happens when you do.

There were also some errors in the code snippets, which is really a consequence of a) the fact that we have to manually keep snippets and playground links in sync, and b) the fact that we don't have typechecking of .svelte snippets (which would have caught the object = { count = 0 } syntax error, for example). We should fix both of those.

[changeset-bot]

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Latest commit: 9429d27

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[pkg-pr-new]

pnpm add https://pkg.pr.new/svelte@14079

commit: 9429d27