Update feature maturity levels to Experimental, Beta, GA

[mintlify]

Summary

• Renames the middle maturity tier from Early Access to Beta across the docs.
• Refocuses Beta and GA definitions in references/workspace/feature-maturity-levels.mdx:
  • Beta — usable, but may change. Expect breaking changes; don't build workflows around it that you can't undo.
  • GA — a promise about stability. The API won't break, the UX won't fundamentally change.
• Updates all callouts/references that linked to the maturity levels page to use the new "Beta" terminology (pre-aggregates, custom granularities, sankey chart, formula table calculations).

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Project	Status	Preview	Updated (UTC)
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