Problem

Dedicated tiered-storage documentation remains after the feature is being removed. Architecture pages, programming-guide links, and "for tiered storage" comments still describe dir_store / WT_STORAGE_SOURCE / flush_tier.

Change

Delete src/docs/tiered-storage.dox, arch-tiered-storage.dox, and custom-storage-sources.dox, and drop the arch-tiered-storage page from dist/docs_data.py. Strip leftover links from programming.dox, extensions.dox, and arch-index.dox. Strip leftover wtperf tiered knobs from wtperf.dox if still present. Keep and edit arch-block.dox (address-cookie objectid is also disagg; do not say it is only tiered), plus glossary / dhandle / prefetch / compact / command-line / file-formats: rewrite "for tiered storage" comments, keep backup:export. Do not edit upgrading.dox (separate notes ticket). Do not edit test/test_coverage.md (owned by WT-18437). Do not change src/ runtime or tests.

Test

cd dist && ./s_all if dist/docs_data.py changes, then cd dist && ./s_fast. No engine behavior change.