[No QA] Update help site docs for Attendees expense table columns

[MelvinBot]

Summary

• Updates help site documentation to reflect the new Attendees and Per attendee columns added to the expense table in feat: Person Amount and Attendees in Expense Table #85332.
• Adds the new columns to the expense table column list in Managing-Expenses-in-a-Report.md.
• Adds a new section to Adding-Attendees.md explaining how attendees appear in the expense table.

Test plan

• Verify the Adding-Attendees.md article renders correctly on the help site
• Verify the Managing-Expenses-in-a-Report.md article renders correctly on the help site
• Confirm column names match the UI labels exactly

Co-authored-by: nkdengineer nkdengineer@users.noreply.github.com

[OSBotify]

A preview of your ExpensifyHelp changes have been deployed to https://e19a1553.helpdot.pages.dev ⚡️

Updated articles:

• articles/new-expensify/reports-and-expenses/Adding-Attendees
• articles/new-expensify/reports-and-expenses/Managing-Expenses-in-a-Report

[melvin-bot]

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cc @muttmuure

[melvin-bot]

@muttmuure Please copy/paste the Reviewer Checklist from here into a new comment on this PR and complete it. If you have the K2 extension, you can simply click: [this button]