[No QA] Docs updates for E/A#92841 clarify when to lock an account

[MelvinBot]

Summary

Updates the New Expensify Report Suspicious Activity help article to match the in-product copy changes shipped in #92841, which reworked the Lock Account page to guide people to the right resolution instead of locking for every security concern.

Changes:

• Adds a "when to lock vs. dispute" section mirroring the new in-product guidance — lost card / unfamiliar charges should be disputed (no lock), while unauthorized account access should be locked.
• Documents the renamed primary button Lock my account (the menu row that opens the page is still Report suspicious activity).
• Notes that the Expensify security team follows up from risk@expensify.com after locking.
• Restructures the article to comply with docs/HELP_AUTHORING_GUIDELINES.md (single # title plus # FAQ, task-based ## headings) and docs/HELPSITE_NAMING_CONVENTIONS.md (exact UI button labels in sentence case).

UI labels were verified against src/languages/en.ts (the product's source-of-truth strings) and the LockAccountPage/SecuritySettingsPage components.

Related code PR: #92841

What did not change

The Expensify Classic Report Suspicious Activity article was left unchanged — PR #92841 only updated the New Expensify Lock Account page.

@Christinadobrzyn, please review the files changed and confirm they reflect the current behavior. Then mark this PR Ready for review.

[OSBotify]

A preview of your ExpensifyHelp changes have been deployed to https://54a42208.helpdot.pages.dev ⚡️

Updated articles:

• articles/new-expensify/settings/Report-Suspicious-Activity