Improve documentation on webhook configuration

Description

Similar to craftcms/commerce-stripe#368, the documentation on webhooks lacks a couple of important details.

Selecting all events as recommended results in Stripe showing a warning that this is inefficient:

I don't want to have to beef up the server just to handle tons of webhooks requests that are irrelevant, and it does looks like there is a lot of events that we don't need.

After selecting all events, the next screen informs you that the selection requires two different payload types:

If you proceed with this selection, you end up creating two endpoints with two separate signing keys. But you can only configure a single signing key in the plugin, so this doesn't work at all.

I assume you need to deselect everything that uses the thin payload style. But that's mostly guesswork.

Suggested fix

The README should maintain a list of which events, or at least which event categories, need to be selected for the integration.

Additional info

• Craft CMS version: 5.9.20
• Stripe for Craft Commerce version: 5.6.2

Start in the README and review the webhook configuration guidance for Craft CMS 5.9.20, Stripe for Craft Commerce 5.6.2, and the related commerce-stripe#368 issue. Document the required event categories, identify events that use thin payloads, and explain how to configure one compatible endpoint and signing key without selecting irrelevant events.