Revise Entra report header with new exclusion info and documentation name

[schrolla]

🗣 Description

Changes the header in the CreateReport.psm1 module that generates the header of the Entra HTML report so no longer references the README file, but rather the overall documentation. Also adds a sentence to better explain what an exclusion is and why it is useful.

💭 Motivation and context

The Entra report should provide up-to-date and accurate information for users to understand their options and point to the current documentation.

Closes #1457

🧪 Testing

Run Invoke-SCuBA -p aad with valid credentials to generate an Entra HTML report. View the report and verify that the new language is visible, correct, and the new link works.

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✅ Post-merge checklist

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[tkol2022]

Slightly tweaked the first sentence so that it matches up with the language we have in the config file documentation. Also we don't support exclusions for roles. Only users and groups can be excluded from policies.
Nice work.