Add summary of each backend to docs

[RossKen]

Type of PR

• BUG
• FEAT
• MAINT
• DOC

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

N/A

Give a brief description for the solution you have provided

One of the main questions we get is what backend people should use, so we should probably have the information in the docs to link to.

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: -20.7%

	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
1797	2023-07-03	16:56:17	1.48641	1.48619	(detached head)	f14b9dd	Intel(R) Xeon(R) Platinum 8272CL CPU @ 2.60GHz	2.5939 GHz	f14b9dd

Test: test_2_rounds_1k_sqlite

Percentage change: -14.3%

	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
1799	2023-07-03	16:56:17	3.65598	3.6499	(detached head)	f14b9dd	Intel(R) Xeon(R) Platinum 8272CL CPU @ 2.60GHz	2.5939 GHz	f14b9dd

Click here for vega lite time series charts

[ThomasHepworth]

This is wonderful, thanks!

Some minor comments and then you're welcome to merge away.