Update Tavern docker image documentation

[google-labs-jules]

Updated docs/_docs/admin-guide/tavern.md to reference spellshift/tavern:latest instead of kcarretto/tavern:latest and updated the link to https://hub.docker.com/r/spellshift/tavern.
Updated docs/_config.yml to set dockerhub to spellshift.

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PR created automatically by Jules for task 6924182917676056799 started by @Cictrone

[google-labs-jules]

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[hulto]

yeah get him out of here 😤

[github-actions]

Summary

Tests 📝	Passed ✅	Failed ❌	Skipped ⏭️	Other ❓	Flaky 🍂	Duration ⏱️
3053    ±0	3053    ±0	0    ±0	0    ±0	0    ±0	0    ±0	1ms    ±0

Previous Results

Build 🏗️	Result 🧪	Tests 📝	Passed ✅	Failed ❌	Skipped ⏭️	Other ❓	Flaky 🍂	Duration ⏱️
#501	✅	3053	3053	0	0	0	0	29.7s

Insights

Average Tests per Run	Total Flaky Tests	Total Failed	Slowest Test (p95)
3053	0	0	10.2s

Slowest Tests

Test 📝	Results 📊	Duration (avg) ⏱️	Duration (p95) ⏱️
eldritch: random::string_impl::tests::test_string_length	3	6.9s	10.2s
eldritch: random::string_impl::tests::test_string_length	3	6.9s	10.2s
eldritch: random::string_impl::tests::test_string_length	3	6.9s	10.2s
eldritch: runtime::tests::input_params	3	3.2s	9.6s
eldritch: runtime::tests::input_params	3	3.2s	9.6s
eldritch: runtime::tests::input_params	3	3.2s	9.6s
eldritch: runtime::tests::http_bindings	3	3.2s	9.5s
eldritch: runtime::tests::http_bindings	3	3.2s	9.5s
eldritch: runtime::tests::http_bindings	3	3.2s	9.5s
eldritch: runtime::tests::multi_print	3	3.2s	9.5s

_{🎉 No failed tests in this run. | 🍂 No flaky tests in this run.}

Github Test Reporter by CTRF 💚