docs: added documentation for data mapping of SAML attributes

[deepakprabhakara]

Related Issue or Design Document

Checklist

• I have read the contributing guidelines and signed the CLA.
• I have referenced an issue containing the design document if my change introduces a new feature.
• I have read the security policy.
• I confirm that this pull request does not address a security vulnerability.
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• I have added tests that prove my fix is effective or that my feature works.
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Further comments

[CLAassistant]

All committers have signed the CLA.

[vinckr]

this looks great 🎉
I have a few small change requests for a bit cleaner style, let me know what you think

[deepakprabhakara]

@vinckr This is ready for another review, thank you.

[vinckr]

lgtm!