10.x: the booking walkthrough and its sample (port of #418) #423

Port of #418 (PR #421) to release/10.x, so the 10.x docs and the 10.3.0 release carry the walkthrough. Cherry-pick with -x once #421 is on main, in a PR against release/10.x that links #421.

What the port changes from main

• samples/Bookings/: no change. Since b0137e2 the sample commits the newest release on nuget.org on net10.0, with a migration from the EF Core 10 tools, so it's the same on both lines. The smoke test retargets its copy to what the branch ships. On release/10.x, check that TheSample_NamesTheNewestReleaseOnNuGet reads that branch's PackageValidationBaselineVersion.
• Integration tests: Microsoft.AspNetCore.TestHost at 10.x, in the integration test project.
• docs/walkthrough.md: links to tree/release/10.x/samples/Bookings, not tree/main/…, so the 10.x site points at the 10.x code.
• test/consumer-smoke-test.sh: 10.x still runs the ComplexIndexes consumer, so expect a conflict in the consumer list. Keep both.
• The menu: none of the site config goes into the port. site/ stays on main, and since fix: leave a page that isn't on release/10.x out of that line's menu #422 the 10.x menu shows the page as soon as the file is on release/10.x.
• CHANGELOG: the 10.3.0 section takes the same Docs: line.

Done when

• The 10.x PR is green, including the smoke consumer under CI=true.
• After it merges, the Docs run on main (triggered by release/10.x's .NET run) shows the walkthrough on https://caffeinatedcoder.github.io/CodoMetis.ValueRanges/10.x/docs/walkthrough.html. This is also the first real test of that trigger.
• Close this issue by hand: Closes acts only on main.