docs: Add extra reducers tutorial link to allow many reducers style guide

[emseidov]

I was reading the style guide the other day and noticed that "Allow Many Reducers to Respond to the Same Action" wasn't clear enough, especially for those who are not familiar with the never APIs like extraReducers and createSlice. Linking the extra reducers tutorial here, I think will be useful.

• :book: New/Updated Documentation Content

[codesandbox-ci[bot]]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

[netlify[bot]]

✅ Deploy Preview for redux-docs ready!

Name	Link
🔨 Latest commit	a99075579b10461d28e0cb0875fbdb99e081326b
🔍 Latest deploy log	https://app.netlify.com/sites/redux-docs/deploys/66c0c4c9016c860008d68d68
😎 Deploy Preview	https://deploy-preview-4733--redux-docs.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[markerikson]

I've actually mostly tried to not add links to the style guide, in contrast to the other docs page.

Could be worth reconsidering, but for now let's hold off on that.