docs: describe the ValidGen generator pipeline

[leocavalcante]

Summary

• Documents the current path from parser.ExtractStructs through the analyzer, code generator, and pkgwriter.Writer.
• Links docs/internals.md from the README and records where TestGen sits relative to the CLI.

Fixes #45

Test plan

• Checked relative links in README.md and docs/internals.md resolve to files in the tree.
• Checked package paths and function names in the new doc against the current source.
• CI make unittests and make endtoendtests on this docs-only change.