* docs(odm): add the on-demand migration operations guide
Positioning against replication, site replication, tiering and the
replication read-proxy; the admin API walkthrough; the full config field
table with defaults and bounds; the semantics table with the test that
pins each row; provider presets and least-privilege source permissions;
integrity, ETag and metadata mapping; protections, error codes,
observability with PromQL, troubleshooting, known limitations and
security notes.
Adds docs/architecture/background-services-inventory.md with the
write-back pipeline, backfill job and backfill recovery loop rows, and
registers it in the architecture index.
Refs rustfs/backlog#2160, rustfs/backlog#2147.
* docs(odm): note migration sources on the replication egress guard
Refs rustfs/backlog#2160.
Zhengchao An