docs: add refine with existing projects guide

[BatuhanW]

Added a new guide in documentation, to show how to use Refine with existing applications.

We have a simple headless example and another one for using refine under /refine route of existing application with Ant Design and React Router.

[changeset-bot[bot]]

⚠️ No Changeset found

Latest commit: d3ed1ff0dc323f39436e866e33f75902c2f1750b

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[nx-cloud[bot]]

☁️ Nx Cloud Report

CI is running/has finished running commands for commit d3ed1ff0dc323f39436e866e33f75902c2f1750b. As they complete they will appear below. Click to see the status, the terminal output, and the build insights.

📂 See all runs for this CI Pipeline Execution

---

✅ Successfully ran 2 targets

• lerna run test --stream --scope @refinedev/* --scope create-refine-app
• lerna run build --ignore=monorepo-* --ignore with-nx --scope @refinedev/* --scope create-refine-app

---

Sent with 💌 from NxCloud.

[netlify[bot]]

✅ Deploy Preview for app-crm-minimal ready!

Name	Link
🔨 Latest commit	5e3c9331dfa5207ad8e7618ebe22f0b38400c786
🔍 Latest deploy log	https://app.netlify.com/sites/app-crm-minimal/deploys/65ae89e5f1743e000876b71c
😎 Deploy Preview	https://deploy-preview-5521--app-crm-minimal.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.