docs: add devtools v3 compatibility note

[mrleblanc101]

fix #2919

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[mrleblanc101]

@posva I'm a noob when it comes to this, I have no idea how to test.
I tried npm link, but couldn't figure it out exactly.
So I edited the pinia.mjs file directly in my browser and it seems to work.

Here is the output:

I confirm I see the message when using v6, and do not see the message when using v7.
The code for isLegacyVueDevTools comes from here: https://github.com/vuejs/devtools/blob/9462f0050c1dc06df56e0144b424acf26cbb5d3a/packages/devtools-kit/src/compat/index.ts#L4