docs: write Migrate from ENS Subgraph guide

[shrugs]

We deleted the placeholder docs/integrate/migrate-from-subgraph.mdx page from the docs site (it was a "Coming Soon" stub). Capturing this as a follow-up so we don't lose track of it.

Scope

Write a real Migrate-from-Subgraph guide for docs/integrate/migrate-from-subgraph.mdx:

• why to migrate (reach the audience still on the legacy ENS Subgraph)
• side-by-side query mapping: common Subgraph queries → ENS Omnigraph equivalents
• step-by-step migration instructions, including how to use the Subgraph-compatible GraphQL endpoint as an intermediate stop
• how ensjs / ens-app-v3 consumers should think about the transition

When the page lands, re-add the sidebar entry under docs/ensnode.io/config/integrations/starlight/sidebar-topics/integrate.ts.

Related

Parent: #1360

[coderabbitai]

🔗 Related PRs

#1942 - Initial docs for ENSDb [merged]
#2051 - docs: rearrange docs structure and prepare for future outline [merged]
#2099 - docs: ENSv2-focused refinements [merged]
#2106 - Omnigraph Quickstart & Walkthrough & Example Apps & ENSv2 Readiness [merged]
#2121 - docs refinements [merged]

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