Assistant message tiers

[ethanpalm]

Documentation changes

This PR updates the assistant docs based on recent updates to assistant message tier billing.

Closes

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For Reviewers

When reviewing documentation PRs, please consider:

✅ Technical accuracy

• Code examples work as written
• Commands and configurations are correct
• Links resolve to the right destinations
• Prerequisites and requirements are accurate

✅ Clarity and completeness

• Instructions are clear and easy to follow
• Steps are in logical order
• Nothing important is missing
• Examples help illustrate the concepts

✅ User experience

• A new user could follow these docs successfully
• Common gotchas or edge cases are addressed
• Error messages or troubleshooting guidance is helpful

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Note

Updates assistant docs to reflect new billing model and configuration UI.

• Introduces tiered message allowances with carryover (up to 50%) and clarifies default no-overage behavior with optional overages
• Rewrites configuration guidance to reference the "Assistant Configurations" page; removes outdated configuration image/frame
• Refines billing instructions: renames sections (Upgrade/Downgrade), clarifies immediate allowance changes, prorated upgrades, next-cycle pricing for downgrades, and that unused messages don’t carry over
• Clarifies where to enable overages in Billing Controls and keeps usage alert guidance

^{Written by Cursor Bugbot for commit 357a2e8. This will update automatically on new commits. Configure here.}

[ethanpalm]

The broken link checker is for the removed images in translated files. It will be fixed when the localization bot runs after this PR is merged.