refactor(docs): align filesystem paths with sidebar navigation groups

Move engine, profile, and index pages out of data-modeling/ into
dedicated directories that match their sidebar group labels in
oxidoc.toml, eliminating the mismatch between URLs and nav labels.

- docs/storage-engines/  — vector, graph, document, columnar, kv, fts, crdt
- docs/columnar-profiles/ — timeseries, spatial
- docs/indexes/           — btree, csr, hash, hnsw, inverted, rtree
- docs/data-modeling/     — now only collections and schemas-types

Update oxidoc.toml sidebar paths and fix an internal cross-link in
the CRDT page. Update README directory tree to reflect new layout.

Author: farhan-syah