[FEATURE] SHIP-0040: Document runtimeClassName field in API reference

[adambkaplan]

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Update API reference documentation to include the runtimeClassName field for Build and BuildRun objects. Part of SHIP-0040.

Describe the solution that you would like.

Overview

This task updates the API reference documentation to document the new runtimeClassName field that allows builds to run with alternative container runtimes.

Description

The API reference documentation needs to be updated to include:

• runtimeClassName field in Build API reference
• runtimeClassName field in BuildRun API reference
• Documentation of precedence behavior (BuildRun overrides Build)
• Field description, type, and example usage

Acceptance Criteria

• Build API reference includes runtimeClassName field
• BuildRun API reference includes runtimeClassName field
• Precedence behavior documented
• Examples provided showing both Build and BuildRun usage

Dependencies

• Blocked by: shipwright-io/build 2074 (API must exist first)

Technical Notes

• The field should be documented as optional (string type)
• Examples should show both Build-level and BuildRun-level usage
• Should reference Kubernetes RuntimeClass documentation for context
• Should note that BuildRun values override Build values

Describe alternatives you have considered.

n/a

Anything else?

Geneated-by: Cursor