Add quickstart guide for skill creators

Adds a tutorial-style quickstart guide that walks the reader through
creating their first Agent Skill — a `roll-dice` skill that teaches an
agent to roll dice using true system randomness. The guide covers
creating the `SKILL.md` file, verifying discovery via `/skills` in VS
Code, testing with a "Roll a d20" prompt, and a brief explanation of the
discovery/activation/execution lifecycle. Includes both bash and
PowerShell command variants, and a note about model variation in
tool-use reliability.

Also adds the quickstart as the first page under "For skill creators" in
`docs.json` navigation.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/docs.json

@@ -22,6 +22,7 @@
       {
         "group": "For skill creators",
         "pages": [
+          "skill-creation/quickstart",
           "skill-creation/best-practices",
           "skill-creation/optimizing-descriptions",
           "skill-creation/evaluating-skills",