Add comprehensive Hex documentation with 14-day visibility limitation

[devin-ai-integration]

Add comprehensive Hex documentation with 14-day visibility limitation

Summary

Replaces the placeholder Hex documentation (which was just a card linking to a Tally form) with comprehensive user-facing setup documentation. The new documentation follows the same structure as other BI tool integrations (like Sigma) and includes the critical 14-day visibility limitation as specified in the Linear ticket ELE-5071.

Key additions:

• Complete API token creation instructions with link to official Hex docs
• Step-by-step connection setup in Elementary's Account settings
• Limitations section highlighting both dataset exclusion and 14-day visibility requirement
• Consistent formatting with other BI tool documentation

The 14-day limitation is based on the days_back: int = 14 parameter found in HexQueryHistoryReader.read_hex_queries_from_history() in the elementary-internal codebase.

Review & Testing Checklist for Human

• Verify the 14-day limitation is technically accurate - Check that HexQueryHistoryReader implementation actually uses 14 days and that this matches the user-visible behavior
• Test the setup instructions end-to-end - Follow the API token creation and Elementary connection steps to ensure they work with the current Hex and Elementary UIs
• Check the API token documentation link - Verify the Hex docs link is current and leads to the correct instructions

Notes

• This addresses Linear ticket ELE-5071 requested by Yosef Arbiv (@arbiv)
• Link to Devin session: https://app.devin.ai/sessions/cf72e534d2054c1d8bf2857f84861d52
• The documentation structure was modeled after /cloud/integrations/bi/sigma.mdx as requested
• No code changes, only documentation updates

[devin-ai-integration]

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[linear]

ELE-5071 add hex docs

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✨ Finishing touches

🧪 Generate unit tests

• Create PR with unit tests
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• Commit unit tests in branch ele-5071-1758098454-add-hex-docs

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