Docs updates for E/A#86177 Add inline category creation from expense flow

[MelvinBot]

Summary

• Documents the new ability for Workspace Admins to create a category directly from the category picker when creating or editing an expense
• Updates the Create-expense-categories.md help site article with a new section and updated keywords
• This feature was added in Feat: add a new category within the category list #86177

Test plan

• Verify the new section renders correctly on the help site
• Confirm the steps match the actual UI flow introduced in Feat: add a new category within the category list #86177

[MelvinBot]

The dryRun failure is not caused by this PR's code changes — it's a transient GitHub infrastructure issue.

The git fetch step failed with GnuTLS recv error (-24): Decryption has failed (a TLS network error on the CI runner) before the code was even checked out. The error message about generateTranslations.ts is a generic fallback that always prints when the dryRun job fails, regardless of the actual cause.

Fix: Re-run the failed job. I don't have permissions to trigger re-runs, so please click "Re-run failed jobs" on the failed workflow run.