docs: link getting started structure

[TomCools]

This resolved 2 items:

• We made changes to the structure of the quickstart repo. This aligns those guides again with the links. Contra PR: chore: clearer getting started output timefold-quickstarts#1108)
• Added a missing segment at the end of the Service Getting Started so the end result matches what is in the quickstart repo.

[Copilot]

Pull request overview

Updates the Quickstart documentation to match the new timefold-quickstarts repository structure and ensures the Service Getting Started guide ends with the missing OpenAPI-spec generation step.

Changes:

• Simplified the shared “Solution source code” section to point directly at the relevant solution project on GitHub via {framework-quickstart-url}.
• Updated the Service Getting Started guide to reuse the shared solution section and added instructions for generating the initial OpenAPI spec (-Dupdate-api).
• Repointed quickstart URL attributes (and added :service-quickstart-url:) to the new getting-started/... directory layout in the quickstarts repo.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
docs/src/modules/ROOT/pages/quickstart/shared/_solutionsourcecode.adoc	Reworks the shared solution-reference section to link to the framework-specific quickstart repo location.
docs/src/modules/ROOT/pages/quickstart/service/getting-started.adoc	Switches to the shared solution section, sets a framework URL, and adds OpenAPI spec generation steps near the end.
docs/src/modules/ROOT/pages/_attributes.adoc	Updates quickstart link attributes to the new quickstarts repo structure and adds the service quickstart URL attribute.

[triceo]

LGTM when Copilot comments resolved.