Update indexers.mdx - added Zerion API

[vshamanov]

Added a link to Zerion API, which supports Abstract (source: https://developers.zerion.io/reference/supported-blockchains)

[cursor]

PR Summary

Low Risk
Single documentation-only change that adds an external link; no code or runtime behavior is affected.

Overview
Adds a new Card link for Zerion API (https://zerion.io/api) to ecosystem/indexers.mdx, expanding the listed data/indexing providers for Abstract.

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