Add docs contribution info + legacy docs/xstate.js.org upkeep info

[laurakalbag]

This PR just updates the contribution guide with some info about how to contribute to our docs and how the “legacy” docs and xstate.js.org docs + landing page are maintained.

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Latest commit: 0d7e101

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[linear]

STA-3959

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Sandbox	Source
XState Example Template	Configuration
XState React Template	Configuration

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