Add docs on udfs in sqlite and duckdb

[RobinL]

Type of PR

• BUG
• FEAT
• MAINT
• DOC

Is your Pull Request linked to an existing Issue or Pull Request?

Update DuckDB and Sqlite docs to reflect how to register Python UDFs

[github-actions]

Test: test_2_rounds_1k_duckdb

Percentage change: -33.1%

	date	time	stats_mean	stats_min	commit_info_branch	commit_info_id	machine_info_cpu_brand_raw	machine_info_cpu_hz_actual_friendly	commit_hash
849	2022-07-12	18:40:05	1.89098	1.87463	splink3	c334bb9	Intel(R) Xeon(R) Platinum 8370C CPU @ 2.80GHz	2.7934 GHz	c334bb9
1713	2023-06-11	07:44:32	1.27421	1.25332	(detached head)	ab2aa46	Intel(R) Xeon(R) Platinum 8272CL CPU @ 2.60GHz	2.5939 GHz	ab2aa46

Test: test_2_rounds_1k_sqlite

Percentage change: -22.8%

	date	time	stats_mean	stats_min	commit_info_branch	commit_info_id	machine_info_cpu_brand_raw	machine_info_cpu_hz_actual_friendly	commit_hash
851	2022-07-12	18:40:05	4.32179	4.25898	splink3	c334bb9	Intel(R) Xeon(R) Platinum 8370C CPU @ 2.80GHz	2.7934 GHz	c334bb9
1715	2023-06-11	07:44:32	3.28868	3.28841	(detached head)	ab2aa46	Intel(R) Xeon(R) Platinum 8272CL CPU @ 2.60GHz	2.5939 GHz	ab2aa46

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[RossKen]

These look like helpful additions 👍