[No QA] Docs updates for E/A#98234 Consolidated Travel Billing past due banner

[MelvinBot]

Help site updates for #98234

#98234 adds a new Account > Subscription billing banner (plus red notification dots) for workspace owners whose Consolidated Travel Billing invoice is past due. It has two states:

• Your travel invoice is past due — shown while still within the grace period, naming the date payment is due. Booking still works.
• Travel booking is paused — shown once the grace period ends and booking is blocked until the invoice is paid.

This PR documents that behavior in the existing Fix a Billing Issue article, which already documents the Subscription-page billing banner and its message → action table:

• Added the two new banner messages to the "If the message says… / Do this" table.
• Added a "Why did my payment fail?" FAQ bullet and a new FAQ answer explaining the past-due → paused travel invoice flow.
• Added retrieval keywords.

Verification notes

• Verified the top-level navigation tab is labeled Account (not "Settings") in the live web UI, matching the existing article's "Account > Subscription" convention.
• The two banner titles ("Your travel invoice is past due", "Travel booking is paused") are quoted verbatim from src/languages/en.ts in [NO QA] Show a banner when a Consolidated Travel Billing invoice is past due #98234.
• The Subscription page requires a workspace/billing owner to reach, so it could not be exercised directly in the test session; the wording follows the existing article and the source PR's copy.

Reviewer note on scope

Consolidated Travel Billing is settled independently of the subscription balance, but the banner appears on the same Account > Subscription page and behaves like the other billing banners (message + red dots), so documenting it in Fix a Billing Issue keeps it where users will look. If you'd prefer this covered in the Consolidated Travel Billing articles instead (or in addition), let me know.

@blimpich, please review the files changed and confirm they reflect the current behavior. Then post a new comment saying @MelvinBot mark this PR as ready for review.

[OSBotify]

A preview of your ExpensifyHelp changes have been deployed to https://472de67a.helpdot.pages.dev ⚡️

Updated articles:

• articles/new-expensify/billing-and-subscriptions/manage-your-subscription-and-billing/manage-billing/Fix-a-Billing-Issue