docs: align builder program docs with Forge deprecation

[vikions]

Summary

• Present Spark and Inkworks as the active Ink Builder Programs
• Mark Forge as archived and Echo as closed in the navigation and overview
• Replace the stale active Forge reference on the Echo page with Inkworks
• Clarify that the remaining Forge documentation is retained for historical reference

Context

Forge was superseded by Inkworks in #610, but the Builder Program overview and Echo page still described Forge as an active flagship program.

This update aligns the related documentation while preserving the original Forge page as a historical reference.

Validation

• Validated src/pages/ink-builder-program/_meta.json
• Checked the diff with git diff --check
• Confirmed stale active Forge references were removed outside the archived Forge page