Add documentation for the to-prd skill

docs/to-prd.md

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+Install this skill:
+
+```bash
+npx skills add mattpocock/skills skill=to-prd -y -g
+```
+
+Source: [mattpocock/skills/to-prd](https://github.com/mattpocock/skills/tree/main/to-prd)
+
+## What it does
+
+`to-prd` turns the current conversation context and codebase understanding into a product requirements document.
+
+The important constraint is that it does not interview the user again. It synthesizes what is already known.
+
+## What the PRD includes
+
+The generated PRD includes:
+
+- problem statement
+- solution
+- extensive numbered user stories
+- implementation decisions
+- testing decisions
+- out-of-scope items
+- further notes
+
+The skill also asks the agent to sketch the major modules that need to be built or modified.
+
+## Deep modules
+
+`to-prd` actively looks for deep module opportunities.
+
+A deep module hides meaningful complexity behind a small, stable, testable interface. That matters for agentic development because a good interface gives tests something durable to target.
+
+## How it fits the workflow
+
+```txt
+grill-with-docs → to-prd → to-issues → tdd
+```
+
+Use `to-prd` after the plan and domain language have been resolved. Then use `to-issues` to break the PRD into tracer-bullet implementation issues.
+
+## Pairs well with
+
+- [grill-with-docs](/skills-grill-with-docs), to make sure the context is precise before the PRD is written
+- [to-issues](/skills-to-issues), to turn the PRD into implementation tickets
+- [tdd](/skills-tdd), to implement the resulting issues one behavior at a time