remove unmaintained v4 docs

[with-heart]

This PR removes the unmaintained v4 docs and the associated docs workflow.

As the concepts.md page notes, these docs are no longer maintained. So let's make that a true statement 😁

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Latest commit: 7f1f75057e38012d689a4037dd72e723e6728113

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[with-heart]

Do you happen to have usage data from Plausible for https://xstate.js.org/docs/? That's what the docs/ folder deploys to right?

I'm wondering if maybe it'd make more sense to just redirect that to https://stately.ai/docs/xstate-v4/studio since that's the more recently-unmaintained unmaintained v4 docs. nvm wouldn't prevent links breaking

Reasons for opening this PR:

• docs folder that isn't related to the version of the code it shares a codebase with is potentially confusing for users/contributors
• multiple v4 docs deployments is confusing for everyone (by which I mean me, at a minimum 😁)
• running docs action on every merge has just been wasting resources for 6 months (last time that dir was touched)

Random off-the-top-of-my-head options:

• redirect to newer v4 docs
• move folder to docs repo and redirect https://xstate.js.org/docs/ pages to corresponding pages under "new" url (https://stately.ai/docs/v4-old?)
• create a branch specifically for old docs and their deployment
• do nothing

[davidkpiano]

Let's close this since we should still keep v4 docs for now.