docs(OIDC feature): create documentation for SIOP

[ossdhaval]

General Guidelines

• Start by covering the general definition of the feature in a few lines and then point to the appropriate section of the SPEC. Please refrain from repeating what is there in the SPEC already.
• Try to make the content Janssen Server specific ( e.g. what are the related Janssen Server properties or feature flags, how to change relevant settings using TUI/command line (use the screenshots), recommendations around recommended values, use-cases)
• Follow the documentation contribution guidelines.
• Feel free to refer to content available in Gluu documentation (or any other IDP) if the content doesn't need to be customized for Janssen Server

Document Location

https://docs.jans.io/head/admin/auth-server/openid-features/siop/

Document Description

Need inputs

Suggested sections

Need inputs

[ossdhaval]

@nynymike Need your inputs in the issue description above about what should be the contents of the document

[ossdhaval]

Since we are not actively working on SIOP support, this document is not needed. It should be removed from navigation and codebase.