feat: add guide for authenticating with a script

[Copilot]

Adds a guide for authenticating Node.js scripts with Solid servers using Client Credentials — covering automated pipelines, bots, CI/CD, and server-to-server integrations where browser-based auth isn't available.

• New guide (docs/guides/authenticating_with_a_script.md): Step-by-step walkthrough covering project setup, credential generation (UI and API), and authenticated requests using @inrupt/solid-client-authn-node
• Index update (docs/index.md): Added link to the new guide
• All config via env vars: SOLID_CLIENT_ID, SOLID_CLIENT_SECRET, and SOLID_OIDC_ISSUER are all loaded from environment variables — no hardcoded server URLs that silently fail
• Cross-platform instructions: Separate Bash and PowerShell examples for setting env vars

const CLIENT_ID = process.env.SOLID_CLIENT_ID;
const CLIENT_SECRET = process.env.SOLID_CLIENT_SECRET;
const OIDC_ISSUER = process.env.SOLID_OIDC_ISSUER;

Related:

• chore: improve documentation of client credential authentication options CommunitySolidServer/CommunitySolidServer#2117
• feat: add client credentials as a MUST solid-oidc#245

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[Copilot]

Pull request overview

This PR enhances the Node.js script authentication guide by making all configuration fully environment-variable based and adding cross-platform setup instructions. Previously, the OIDC_ISSUER (Solid server URL) was hardcoded while other credentials were loaded from environment variables, creating an inconsistency. The update makes the guide more production-ready and accessible to developers on different operating systems.

Changes:

• Modified OIDC_ISSUER to load from process.env.SOLID_OIDC_ISSUER instead of using a hardcoded value
• Added cross-platform environment variable setup instructions with separate examples for Bash (Linux/macOS) and PowerShell (Windows)
• Enhanced documentation with clearer guidance on passing all three required environment variables when running the script

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