docs: document conductor.enable.ui.serving for headless deployments

[bishnubista]

Summary

Closes #1074. Documents conductor.enable.ui.serving, the property that controls Conductor's bundled UI serving. Until now, operators wanting to run a headless (API-only) Conductor server had to discover the flag by reading SpaInterceptor.java.

The property toggles two Spring MVC registrations as a pair, both gated on the optional SpaInterceptor bean in RestConfiguration:

1. The HandlerInterceptor that forwards unmatched paths to index.html so the SPA's client-side router can handle them.
2. The resource handler that serves the UI bundle at /static/ui/**.

What changed

• docs/documentation/configuration/appconf.md — new "UI Serving" section with the property reference table, the authoritative path-exclude list, and an example usage block. Cross-linked to the deploy guide.
• docs/devguide/running/deploy.md — new "Headless (API-only) deployment" subsection under Production configuration, including a note that a small placeholder index.html shipped inside the server JAR is still served at / by Spring Boot's default static-resource handling (so the behavior is not a strict 404 at the root).
• docker/server/config/config.properties — commented example of conductor.enable.ui.serving=false near the top of the default Docker config.

How I verified

Every behavioral claim was checked against source:

• Property name and default: rest/src/main/java/org/conductoross/conductor/SpaInterceptor.java lines 25-28 (matchIfMissing = true).
• The two gated registrations: rest/src/main/java/org/conductoross/conductor/RestConfiguration.java
  • Lines 36, 51-66 — SpaInterceptor registration via addInterceptors() is skipped when the bean is absent.
  • Lines 70-76 — /static/ui/** resource handler in addResourceHandlers() is also skipped.
• Authoritative path exclude list: RestConfiguration.java:54-64 (excludePathPatterns(...) at the registration site). The doc lists exactly these patterns, not the belt-and-suspenders checks inside SpaInterceptor.preHandle().
• Existing explicit default: server-lite/src/main/resources/application.properties:11.
• Placeholder at /: rest/src/main/resources/static/index.html — confirmed it is a short page (logo + links to Swagger UI and the documentation), served by Spring Boot's default static-resource handling regardless of the flag.

Verified independently that no other class reads this property (grep for enable.ui.serving and enableUiServing — only SpaInterceptor.java and the server-lite default were hit).

Per the project's documentation policy in CLAUDE.md, no behavior is described that isn't traceable to one of these source files.

Test plan

• mkdocs serve renders the new sections without warnings
• Cross-links resolve in both directions:
  • appconf.md → deploy.md#headless-api-only-deployment
  • deploy.md → appconf.md#ui-serving
• Tables render correctly on the docs site
• docker/server/config/config.properties still loads (it's a comment-only change, but worth running docker compose up to confirm)