docs: Move Join operator to public preview

[dominicchapman]

• Moves join operator to public preview rather than private preview
• Fixes references to "Features states" instead of "Feature states" across docs
• Revises limits for join operator to reflect more recent update (graduated limits, based on field count)

Primary pages to preview are:

1. https://axiom-dominic-join-feature-state.mintlify.app/apl/tabular-operators/join-operator
2. https://axiom-dominic-join-feature-state.mintlify.app/getting-started-guide/feature-states

[kamstrup]

Looks good 👍

Somewhat related to this PR: I think we only support leftouter, inner, and innerunique join types. The docs list all the ones that kusto support.

Adding a few more join types would be 1-2 weeks of work I think. In case we want to commit to it.