docs(v2): add missing doc.go files for fields and log packages

Doug Barrettrequested to merge
db-v2-doc-go into master

Context

The project guidelines state: "Include excellent godocs, including package documentation in doc.go." Several v2 packages were missing these files, which hurts discoverability on pkg.go.dev.

Complementary to !348 (merged) which adds Example_ functions; this MR adds the doc.go package-level documentation files.

What's in this MR?

Adds doc.go files for three v2 packages that were missing them:

  • v2/fields -- canonical log/trace field name constants
  • v2/log -- slog-based structured logger
  • v2/testing/logtest -- in-memory log recorder for tests

The remaining testing/ packages (tracetest, apptest, secrettest) already have package-level doc comments in their source files.

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