docs: add user documentation for 6 merged features

[JacobCoffee]

Summary

• Feature toggles (feat: add feature toggle system for disabling modules and UI #28): New section in configuration.md — settings defaults, per-conference DB overrides, Python API (is_feature_enabled, require_feature, FeatureRequiredMixin), and template context processor usage
• Pretalx overrides (feat: add pretalx override system for JIT talk management #30): New section in pretalx-integration.md — TalkOverride, SpeakerOverride, RoomOverride, SubmissionTypeDefault models, admin workflows, and sync interaction
• Global ticket capacity (feat: add global ticket capacity for conferences #32): New section in registration-flow.md — Conference.total_capacity enforcement in cart add and checkout, plus TOML bootstrap field in configuration.md
• Financial dashboard (feat: add financial overview dashboard for conference management #27): New page management-dashboard.md — revenue summary cards, order/cart/payment breakdowns, ticket type sales table
• Voucher bulk generation (feat: add voucher bulk code generation #26): Section in management-dashboard.md — bulk code form fields, generated code format, transactional guarantees
• Travel grants & activities (feat: travel grant day detection and enhanced registration UI #29): New page programs-activities.md — grant application flow, day detection (schedule-derived + section fallback), grant lifecycle statuses, receipts/payment info, activity types, signup caps, waitlisting

Also adds grid cards on the docs landing page for the two new pages.

Test plan

• make docs builds with no new errors
• make ci passes (1518 tests, lint, fmt, type-check)
• Sphinx Lint pre-commit hook passes
• New HTML pages render in docs/_build/html/

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[Copilot]

Pull request overview

Adds end-user documentation pages/sections for several recently merged features, expanding the Sphinx docs to cover new configuration options, management UI capabilities, and program-related workflows.

Changes:

• Documented global ticket capacity enforcement and Conference.total_capacity configuration.
• Added new documentation pages for Programs & Activities and the Management Dashboard, plus expanded Pretalx integration docs with override workflows.
• Added feature toggle documentation (settings + per-conference DB overrides) and updated the docs landing page to link new pages.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
docs/registration-flow.md	Adds a “Global Ticket Capacity” section explaining capacity rules, enforcement points, locking, and error messages.
docs/programs-activities.md	New page documenting travel grants and activity signup behavior (waitlisting, Pretalx linkage, lifecycle).
docs/pretalx-integration.md	Adds a detailed “Overrides” section describing override models, effective_* access patterns, and admin workflows.
docs/management-dashboard.md	New page describing organizer management UI features (financial overview + voucher bulk generation).
docs/index.rst	Adds grid cards and toctree entries for the two new docs pages.
docs/configuration.md	Documents feature toggles and adds total_capacity to the TOML bootstrap docs, with a link to enforcement details.

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[JacobCoffee]

Addressed both review comments in d9b7b7b:

• Access control (management-dashboard.md): Separated general management access (superuser or change_conference perm) from the financial dashboard's additional Program: Finance & Accounting group access.
• Receipt/payment prereqs (programs-activities.md): Clarified that both ACCEPTED status and a non-zero approved_amount set by a reviewer are required before receipt uploads and payment info submission are unlocked.