[Docs] Document build_subject_identifier_from_sp_config and Console Subject settings for introspection

[nathan20061025]

Purpose

Resolves wso2/product-is#28429

Documents how to control the username format in OAuth2 /oauth2/introspect responses:

• Documents the role and server-wide impact of build_subject_identifier_from_sp_config in deployment.toml.
• Documents the Console Subject settings (Assign alternate subject identifier, Include userstore domain, Include tenant domain).
• Clarifies that both the deployment.toml flag and the Console settings must be configured together.

Related PRs

None

Test environment

Documentation preview / Markdown lint check.

Security checks

• Followed secure coding standards in http://wso2.com/technical-reports/wso2-secure-engineering-guidelines?
• Ran FindSecurityBugs plugin and verified report?
• Confirmed that this PR doesn't commit any keys, passwords, tokens, usernames, or other secrets?

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• en/includes/guides/authentication/oidc/token-validation-resource-server.md

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