chore: linkify provider names and reshape infrastructure table in README

[leggetter]

Summary

• Provider Webhook Skills table: each Provider cell now links to the provider's webhooks docs (URLs sourced from providers.yaml docs.webhooks).
• Webhook Infrastructure Skills table: reshaped to Product | Skill | What It Does to match the provider table. The two Event Gateway rows are disambiguated — one for the gateway product itself, one labelled "(receiver)" for the verification-side skill.
• Webhook Handler Pattern Skills table: left as two columns — these are framework-agnostic (no vendor), so forcing a Product column would mean inventing one.

Test plan

• README renders correctly on GitHub
• All provider links resolve to the right docs page
• Infrastructure links resolve (hookdeck.com/docs, hookdeck.com/docs/verification, outpost.hookdeck.com/docs)

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