Update indexing FAQ and delegating guide for Graph Horizon

[cargopete]

Summary

Closes #1072

Updates the Indexing Overview FAQ and Delegating guide to reflect Graph Horizon (GIP-0066/0068/0085) changes. The Horizon upgrade fundamentally changed how rewards are collected and how delegation works. The primary operational docs had not been updated.

Changes

website/src/pages/en/indexing/overview.mdx

• Rewrite FAQ: rewards are collected via periodic POI submission, not on allocation close
• Remove 28-epoch maximum allocation lifetime language (Horizon removed maxAllocationEpochs)
• Rewrite "The Life of an Allocation" section for Horizon lifecycle
• Update POI FAQ entry to reflect rolling submission model
• Add concrete maxPOIStaleness value (28 days) with force-closure consequences
• Add new "Provisions and Data Services" section
• Add inline notes on per-service delegation cut parameters and flexible slashing
• Add cross-reference banner to /en/graph-horizon/ at top of page

website/src/pages/en/resources/roles/delegating/delegating.mdx

• Remove all references to the 0.5% delegation tax (confirmed removed: __DEPRECATED_delegationTaxPercentage)
• Add "Delegating under Graph Horizon" subsection covering per-data-service delegation
• Document support for up to 100 simultaneous undelegation requests
• Add note on auto-migration of existing delegations to SubgraphService
• Add forward-looking note on future slashability capability

Verified from source

Parameter	Source	Value
maxPOIStaleness	migrate.arbitrumOne.json5	2419200 sec (28 days)
delegationTaxPercentage	HorizonStakingStorage.sol	Deprecated, removed
maxAllocationEpochs	HorizonStakingStorage.sol	Deprecated, removed

What was NOT changed

• /en/graph-horizon/ section (already accurate, used as source of truth)
• GraphTally/TAP guide (already updated for Horizon v2.0.0)
• Curation documentation (Horizon did not change curation mechanics)