Postgres docs

[ADBond]

Type of PR

• BUG
• FEAT
• MAINT
• DOC

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Give a brief description for the solution you have provided

Primarily adding some more detail to the docs on the Postgres backend (both for users and for devs). However in the course of laying out this detail, a few small issues became apparent which I have amended:

• changed when the Splink schema is created, so that UDFs live in that schema rather than public
• user option to specify additional schemas to add to the search path
• drop test user at the end of the testing session
• reduce the database privileges granted to the test user to those minimally required for testing
• postgres setup/teardown scripts executable

PR Checklist

• Added documentation for changes
• Added feature to example notebooks at tutorial in splink_demos (if appropriate)
• Added tests (if appropriate)
• Made changes based off the latest version of Splink
• Run the linter

[github-actions]

Test: test_2_rounds_1k_duckdb

Percentage change: -8.9%

	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
1821	2023-07-06	16:40:30	1.72269	1.70759	(detached head)	787d4df	Intel(R) Xeon(R) Platinum 8171M CPU @ 2.60GHz	2.0952 GHz	787d4df

Test: test_2_rounds_1k_sqlite

Percentage change: -0.8%

	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
1823	2023-07-06	16:40:30	4.24251	4.22609	(detached head)	787d4df	Intel(R) Xeon(R) Platinum 8171M CPU @ 2.60GHz	2.0952 GHz	787d4df

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[RossKen]

This is ace - thank you for putting it together! Just made a small change to fix a relative path for the new backends folder