[docs] Handle integration docs 404

[prakhargupta1]

https://deploy-preview-4475--mui-toolpad-docs.netlify.app/toolpad/core/introduction/integration/ gets redirected.

Fix a regression introduced in #4411

[oliviertassinari]

Adding a 301 is great, we need this.

Now, it's not enough to fix that regression:

toolpad/docs/data/toolpad/core/introduction/base-concepts.md

Line 111 in b67c51f

Now that you understand the basic concepts of Toolpad Core, you're ready to start integrating it into your project. Head over to the [integration docs](/toolpad/core/introduction/integration/) to learn more.

there is no equivalent to this now. I guess next step should link to the tutorial, or maybe we can remove that whole "Next steps" section.

[bharatkashyap]

Adding a 301 is great, we need this.

Now, it's not enough to fix that regression:

toolpad/docs/data/toolpad/core/introduction/base-concepts.md

Line 111 in b67c51f

Now that you understand the basic concepts of Toolpad Core, you're ready to start integrating it into your project. Head over to the [integration docs](/toolpad/core/introduction/integration/) to learn more.

there is no equivalent to this now. I guess next step should link to the tutorial, or maybe we can remove that whole "Next steps" section.

We can remove it entirely 👍🏻 @prakhargupta1 Can you take this up? I'll redeploy once this is merged as well