Add detailed explanation about several key points in Rich Authorization Requests doc

[ShanChathusanda93]

Document issue type

Type/Improvement (Suggest changes to enhance the clarity, usability, or accuracy of existing content)

Description

In [1] we have the RAR doc which contains steps to configure the feature in IS. However several key elementas are missing in this doc.

1. Rich Authorization Requests (RAR) (RFC 9396) enhance authorization mechanisms -> Needs to have the RFC link
2. Step 1: Register a new authorization details type -> Need to explain about what is the spec is mandating and the scenario that the doc is explaning since the doc contains a sample scenario.
3. Step 3: Customize authorization details validation -> Need to explain how this customization works with custom scenarios. We can use the same scenario that we used in the 2nd point. Also if we can add some sample codes to the methods it will be great.
4. At the end of the doc we can also mention how the APIs are working with this authorization details.

[1] https://is.docs.wso2.com/en/latest/guides/authorization/rich-authorization-requests/#configuring-your-application-for-rar

Version

IS 7.1.0