docs: document OpenRPC account selection for integrations

[jcstein]

Summary

• add a new stacks guide that explains how to select a Celestia signer/account via OpenRPC (blob.Submit options)
• include JSON-RPC examples for key_name, signer_address, and state.BalanceForAddress
• add integration notes for OP Stack, Arbitrum Nitro, Rollkit, RollApps, and Sovereign SDK
• link the new guide from OP alt-DA and Nitro DAS pages and add it to stacks navigation

Testing

• not run (eslint is unavailable in this environment)

Fixes #1586

[github-actions]

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Your preview is ready: https://celestiaorg.github.io/docs-preview/pr-2444/

[Copilot]

Pull request overview

Adds a new Stacks documentation guide describing how integrations can select a specific Celestia signer/account via OpenRPC options on blob.Submit, and links it from relevant stack pages.

Changes:

• Add a new /build/stacks/account-selection guide with JSON-RPC examples for blob.Submit and state.BalanceForAddress.
• Link the new guide from the OP alt-DA and Arbitrum Nitro DAS stack docs.
• Add the new page to the Stacks navigation meta.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
app/build/stacks/op-alt-da/introduction/page.mdx	Adds a resource link to the new account selection guide.
app/build/stacks/nitro-das-server/page.mdx	Adds a resource link to the new account selection guide.
app/build/stacks/account-selection/page.mdx	New guide explaining signer/account selection via OpenRPC with examples and integration notes.
app/build/stacks/_meta.js	Adds the new page to Stacks navigation.

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