add banner pointing to google form

[RossKen]

Type of PR

• BUG
• FEAT
• MAINT
• DOC

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

No

Give a brief description for the solution you have provided

Adding a banner to the docs site to a Google form to gather feedback for further developments for the docs site.

PR Checklist

• Added documentation for changes
• Added feature to example notebooks at tutorials 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: -33.9%

	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
1752	2023-06-21	10:02:25	1.24094	1.23879	(detached head)	70344c6	Intel(R) Xeon(R) Platinum 8370C CPU @ 2.80GHz	2.7934 GHz	70344c6

Test: test_2_rounds_1k_sqlite

Percentage change: -18.2%

	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
1754	2023-06-21	10:02:25	3.48788	3.48432	(detached head)	70344c6	Intel(R) Xeon(R) Platinum 8370C CPU @ 2.80GHz	2.7934 GHz	70344c6

Click here for vega lite time series charts

[ThomasHepworth]

LGTM