docs: create hosted collective and incomplete expense payee editing

[cursor]

Summary

Documents two user-facing frontend changes from the week of July 12–19, 2026.

Docs added/updated

• fiscal-hosts/managing-your-collectives/hosted-collectives.md — New Create collective section covering the host-dashboard workflow to create hosted collectives, form fields, admin requirements, Invited badge, invitation flow, and activity log events.
• expenses-and-getting-paid/editing-an-expense.md — Clarifies that expense submitters can edit payee information while an expense is marked incomplete.

Codepaths covered

• Frontend #12260 / API #11896 — Create Hosted Collective (CreateHostedCollectiveModal, createCollective mutation from host dashboard)
• API #11992 — Allow expense submitter to edit payee on INCOMPLETE expenses

Knowledge gaps addressed

• No prior documentation for host admins creating collectives directly from Hosting > Hosted Collectives
• Undocumented Invited badge and post-creation Invite admin fallback
• Missing guidance that incomplete expenses allow payee/payout method edits by the submitter

Validation

• npm run validate passes