[FR]: Expand spark.md and spark-icons.md module stubs

[soulcramer]

Describe the problem

spark/src/spark.md contains "Main library module." (one line). spark-icons/src/spark-icons.md contains "Icons library module." (one line). Neither provides an overview, index, or getting-started content.

Describe the solution

• spark.md: Add module overview, list of component packages, and link to theming docs
• spark-icons.md: Add icon-addition workflow pointer (detailed workflow covered in the dedicated icon docs issue)

Additional context

Priority: Low. Effort: S.