Update free signup and auto-pay FAQ articles to match Pylon support articles

[devin-ai-integration]

Update free signup and auto-pay FAQ articles to match Pylon

Summary

Aligns two FAQ articles with their corresponding Pylon support articles:

Can I sign up for free? (can-i-sign-up-for-free.mdx)

• Simplified free tier description: "process up to $50 in audio" instead of "transcribe up to $50 in transcription and will be able to process up to five files concurrently"
• Updated language: "used the $50 in free credits" instead of "transcribed $50 worth of files"
• Added new "What happens when you exceed the free tier limit?" section with error message, balance management tips, and resolution steps

How does Auto-pay work? (how-does-auto-pay-work.mdx)

• Updated Note to include "or goes negative" scenario
• Added the specific error message users receive when balance is negative

Review & Testing Checklist for Human

• Verify removal of "process up to five files concurrently" from free tier description is intentional (this info was in the old docs but not in the Pylon source article)
• Confirm the Billing page link in the new section is correct — this URL was inferred, not explicitly in the Pylon article
• Preview both pages locally (fern docs dev) to verify rendering, especially the new code block and nested heading structure in the free signup article

Notes

• Source articles: Can I sign up for free? and How does Auto-pay work?
• Requested by: @LeeVaughn
• Link to Devin run

[devin-ai-integration]

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