[docs] Add a new section for Integration

[prakhargupta1]

Preview: https://deploy-preview-4411--mui-toolpad-docs.netlify.app/toolpad/core/integrations/nextjs-approuter/

[prakhargupta1]

@bharatkashyap Not sure why Netlify preview fails. Can you have a look?

[bharatkashyap]

Instead of a root level section, can we make it a sub-section of the Introduction? I think that makes more sense in the hierarchy of the navigation, what do you think?

[apedroferreira]

Some Vale changes in my own copy :D

Also why did we remove the numbers from the headings?
In the React Router guide I sometimes mention "step 1" or "step 3" but this way it doesn't make sense anymore, we'd have to change it.

[prakhargupta1]

Also why did we remove the numbers from the headings?
In the React Router guide I sometimes mention "step 1" or "step 3" but this way it doesn't make sense anymore, we'd have to change it.

Based on the core docs Integration section, I removed the numbers. There were two mentions of step 1, step 2 which I have now tackled.

[apedroferreira]

Thanks Prakhar!

[bharatkashyap]

Instead of a root level section, can we make it a sub-section of the Introduction? I think that makes more sense in the hierarchy of the navigation, what do you think?

We should have also added new redirect rules to the _redirects file given that we went ahead with a new root level section. The old URLs are now returning 404s: https://mui.com/toolpad/core/introduction/integration/

@prakhargupta1 Can you take this up? I'll redeploy the docs once this is merged

[prakhargupta1]

Oh sorry, completely missed it. Thanks for pointing it out! PR: #4475