docs(crossseed): document the Cross-seed instance setup

Parent

#3256

What to build

PR 2. The cross-seed docs page gets a "Cross-seed instance" section that shows a setup with a Source instance (for example the library instance that the arrs manage) and a Cross-seed instance, so that a user can copy it.

Acceptance criteria

• The section explains the route, the required hardlink or reflink mode, the path rows, Test route, and the check before each add.
• It covers these points:
  • The Cross-seed instance's qBittorrent needs read and write access to the mapped folders. It downloads pending files there.
  • The Source instance's blocklist does not apply to routed adds.
  • Users of the export-then-delete workaround can turn those two automations off.
  • NFS users can lower their folder cache time if checks fail often.
  • To swap the roles of two instances, move or delete the old routed torrents before you clear the route.
  • An offline Source instance blocks adds to its Cross-seed instance until the user enables it or clears its route.
• The docs use "Source instance" and "Cross-seed instance" as the terms. "Library" appears only as a description.
• The docs do not describe remote or SSH instances, because the SSH setup screen (#2917) has not shipped. Say that a route needs Local Filesystem Access.
• Prose follows the docs rules in AGENTS.md (simple English, no em dashes).

Rules for every ticket of #3256

• "Unavailable" means one of these: offline, disabled in qui, or (for an add or a directory listing only) older than qBittorrent 5.0 (WebAPI 2.11.2). Use the shared availability check from the route storage ticket. Do not write a new one.
• Tests talk only to local fakes and temp folders, never to a real host. Each new test fails before the change and passes after it.
• No user-facing text mentions SSH. No text calls the Cross-seed instance "remote", because "remote instance" means an SSH instance. An error for an instance that is in remote mode can say "remote instance".
• If the ticket changes the API, update internal/web/swagger and make sure that make test-openapi passes.
• Glossary terms: Cross-seed instance (the instance that receives the cross-seeds found on another instance) and Source instance (an instance that names a Cross-seed instance and holds the matched torrents). Do not say "target instance" or "library instance" for these.
• Delivery: tickets for PR 1 land on one integration branch that becomes PR 1. Tickets for PR 2 land on a second branch that becomes PR 2, after PR 1 merges. Do not merge one ticket to develop alone.