Expand docs for relational selects and conditions in search_by_conditions

Expand documentation around how relational selects and conditions work for `search_by_conditions`. The current treatment is buried and not comprehensive.

## Where docs live today

- [`/docs/reference/resources#select`](https://docs.harperdb.io/docs/reference/resources#select) — has the underlying primitives but is hard to find from operations-api context
- [`/docs/developers/operations-api/nosql-operations#search-by-conditions`](https://docs.harperdb.io/docs/developers/operations-api/nosql-operations#search-by-conditions) — lacks examples that show relational selects / cross-table conditions

## Ask

- Add a "Relational selects with `search_by_conditions`" section to the operations-api docs, with multiple worked examples (one-to-many, many-to-many, nested selects, conditions referencing related-table attributes).
- Link the reference/resources `#select` material from the operations-api page so the relationship between the two is obvious.

## Acceptance criteria

- Operations-api `search-by-conditions` page contains a relational examples section with at least 3 worked end-to-end examples (input JSON, expected response shape).
- Cross-links between the operations-api and reference/resources pages exist.

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Tracked in Jira: [CORE-3033](https://harperdb.atlassian.net/browse/CORE-3033)

🤖 Filed by Claude on behalf of Kris.

[CORE-3033]: https://harperdb.atlassian.net/browse/CORE-3033?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ