Migrate asset references from crate-clients-tools repository

## Summary

The documentation currently references assets (logos, screenshots, diagrams) from the `crate-clients-tools` repository, which appears to no longer exist. While these references currently work (likely through redirects), we should migrate these assets to a more stable location.

## Context

This issue was raised during review of PR #369:
- PR: https://github.com/crate/cratedb-guide/pull/369
- Comment: https://github.com/crate/cratedb-guide/pull/369#discussion_r2405278279
- Raised by: @bmunkholm

## Affected Files

The following files contain references to `github.com/crate/crate-clients-tools/assets/*`:

### Topic Documentation (3 files, 3 references)
- `docs/topic/visualize/index.md` - Plotly logo
- `docs/topic/framework/index.md` - Plotly and Streamlit logos (2 refs)

### Integration Documentation (19 files, 37 references)
- `docs/integrate/hop/index.md` - Apache Hop screenshots (3 refs)
- `docs/integrate/mindsdb/index.md` - MindsDB logo
- `docs/integrate/prometheus/index.md` - Prometheus logo and architecture diagram (2 refs)
- `docs/integrate/telegraf/index.md` - Telegraf logo
- `docs/integrate/superset/index.md` - Dashboard screenshot and Preset Cloud logo (2 refs)
- `docs/integrate/sql-server/index.md` - Microsoft and MSSQL logos (2 refs)
- `docs/integrate/streamlit/index.md` - Streamlit logo
- `docs/integrate/pyviz/index.md` - Datashader examples (2 refs)
- `docs/integrate/locust/tutorial.md` - Link reference
- `docs/integrate/pycaret/index.md` - PyCaret logo
- `docs/integrate/node-red/index.md` - FlowFuse logo
- `docs/integrate/plotly/index.md` - Plotly logo and multiple screenshots (7 refs)
- `docs/integrate/nifi/index.md` - NiFi screenshots (3 refs)
- `docs/integrate/mlflow/index.md` - MLflow logo
- `docs/integrate/meltano/index.md` - Meltano logo
- `docs/integrate/cluvio/index.md` - Cluvio dashboard screenshots (2 refs)
- `docs/integrate/datashader/index.md` - Datashader examples (2 refs)
- `docs/integrate/dask/index.md` - Dask logo
- `docs/connect/cli.md` - CLI screenshots (3 refs)
- `docs/connect/mcp/index.md` - Discussion link

### Other (1 file)
- `backlog.md` - Already mentions this migration task

**Total: 23 files with 40+ asset references**

## Recommended Actions

1. Identify a suitable location for these assets (e.g., `cratedb-guide` repository itself, or a dedicated assets repository)
2. Download and migrate all referenced assets
3. Update all documentation files with new asset URLs
4. Verify all assets render correctly
5. Consider adding a policy for where to host documentation assets in the future

## Related

- Related to backlog item: "Migrate all content from `crate-clients-tools`"