Refactor existing "Compare roles"-script and update readme to link to OOTB functionality

[WillemZeiler]

PR Description:

This PR contains 2 improvements:
To me the most important one is that in the readme we now refer to OOTB functionality. But I decided to improve the script while I was at it as well.

Update compare-roles-2-users.js

Refactored the script making the followingimprovements:

• username_a and username_b renamed to usernameA and usernameB for consistent camelCase naming on lines 6–7.
• set_a and set_b renamed to rolesUserA and rolesUserB for clearer intent on lines 9–10 and 44–45.
• Duplicate role-fetching logic for each user replaced with reusable getUserRoles() function on lines 12–24.
• Role uniqueness ensured using indexOf() check inside getUserRoles() on line 22.
• Role comparison logic using concat() and filter() replaced with explicit difference() and intersection() functions on lines 26–39.
• gs.print() replaced with gs.info() for consistent platform logging on lines 47–49.
• Inline comments added to explain configuration, function purpose, and output sections throughout the script.

Update README.md

Updated the readme to reflect the changes to the script. Additionally, included the link to Access Analyzer which provides Role comparison in the UI OOTB

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[Paulsylo25]

Looks good for me! Thanks for your contribution