docs(duckdb-geospatial): document adding R-Tree spatial index with raw_sql

[ncclementi]

Is your feature request related to a problem?

No

What is the motivation behind your request?

DuckDB added support for spatial indexes in their latest release 1.1.0 which will enable faster spatial joins.
See https://duckdb.org/2024/09/09/announcing-duckdb-110.html#r-tree

Describe the solution you'd like

I'd like Ibis to have a way to support the creation of this type of index. I'm not sure if this is possible but I'd like to discuss how we would implement this in the comments of this issue.

What version of ibis are you running?

main

What backend(s) are you using, if any?

DuckDB

Code of Conduct

• I agree to follow this project's Code of Conduct

[gforsyth]

As per some internal discussion, we may not want to open up the huge can of worms that represents support for creating indexes, but we should, at the least, document how a user can go about both creating those indexes using raw_sql and also checking whether an existing table HAS an index on it (as a pre-check before doing an otherwise more expensive spatial join)

[ncclementi]

I'm happy to close this as not planed, unless we want to leave it open for some reason.

[gforsyth]

I would leave it open for now and close it out with a PR adding a how-to doc on using raw_sql to add the index and how to check that an rtree index has been added.