Add docs for identity provider in drasi-server

[ruokun-niu]

Description

This pull request adds a comprehensive new documentation page that explains how to configure and use identity providers in Drasi Server. The page covers the structure of the identityProviders configuration, details for each supported provider type (password, Azure, AWS), and validation rules. This documentation will help users securely manage authentication for sources and reactions without hardcoding credentials.

The most important changes include:

New documentation for identity providers:

• Added a new guide at docs/content/drasi-server/how-to-guides/configuration/configure-identity-providers/_index.md that explains how to declare reusable identity providers, reference them from sources and reactions, and configure provider-specific authentication methods.

Provider-specific configuration instructions:

• Detailed instructions and YAML examples for the built-in password provider, Azure provider (with all supported authentication methods), and AWS provider (including ambient credentials, role assumption, and IRSA on EKS).
• Tables listing required and optional fields for each provider type, with explanations of each field’s purpose.

Usage and validation guidance:

• Clarified how identity providers are referenced in sources and reactions, including multi-instance mode scoping rules.
• Listed validation rules and error conditions for identity provider configuration, such as required fields, uniqueness, and plugin registration requirements. (docs/content/drasi-server/how-to-guides/configuration/configure-identity-providers/_index.mdR1-R215

Type of change

• This pull request fixes a bug in Drasi and has an approved issue (issue link required).
• This pull request adds or changes features of Drasi and has an approved issue (issue link required).
• This pull request is a minor refactor, code cleanup, test improvement, or other maintenance task and doesn't change the functionality of Drasi (issue link optional).

Fixes: #issue_number

[Copilot]

Pull request overview

Adds a new Drasi Server configuration guide describing how to declare reusable identityProviders and reference them from sources/reactions, including provider-specific YAML examples and field reference tables.

Changes:

• Added a new documentation page for identity provider configuration and usage.
• Documented built-in password provider and plugin-based azure / aws providers (auth methods, fields, examples).
• Added a “Validation rules” section describing configuration constraints and common failure cases.

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