docs: add Envio to EVM indexer providers

[Jordy-Baby]

Summary

Adds a new page at evm/indexer-providers/envio covering Envio's three products (HyperIndex, HyperSync, HyperRPC), with a Sei-specific config.yaml snippet and a link to the Sei USDC indexing tutorial.

One-line addition to docs.json adds the page to the Indexers nav group.

Notes

• Mirrors the structure and frontmatter style of the existing provider pages (Goldsky, The Graph, etc.)
• Sei chain ID and HyperSync/HyperRPC endpoints for mainnet (1329) and testnet (1328) are included in the endpoints table

[cursor]

PR Summary

Low Risk
Documentation-only changes with no runtime, auth, or application code impact.

Overview
Adds Envio as a new EVM indexer provider page and wires it into the Mintlify Indexers nav (listed first under that group in docs.json).

The new evm/indexer-providers/envio doc explains HyperIndex, HyperSync, and HyperRPC, when to use each, Sei mainnet endpoints (chain ID 1329, HyperSync/HyperRPC URLs), testnet links, a minimal config.yaml example for Sei, and links to Envio quickstarts and support channels—aligned with the style of existing provider pages like Goldsky.

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