Added migration guide from 3 to 4 to docs

website/docs/getting-started/custom-theme.md

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 id: custom-theme
 title: Custom theme
-sidebar_position: 4
+sidebar_position: 5
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 We've made hundreds of custom components across dozens of projects, including complete custom themes. Most of them began as source forks of `uniforms-unstyled` - one simply copies the source and imports `./some/project/path/uniforms-custom-theme-with-a-cool-name`.

website/docs/getting-started/faq.mdx

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 id: faq
 title: FAQ
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+sidebar_position: 6
 ---
 
 ### How can I customize/style my form fields?

website/docs/getting-started/migrating-3-to-4.md

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+---
+id: migrating-4-to-4
+title: Migrating v3 to v4
+sidebar_position: 4
+---
+
+This guide is designed to help you through the migration. If you went through it and encountered any problems - do let us know. For more information on why certain changes were made, see the [CHANGELOG.md](https://github.com/vazco/uniforms/blob/master/CHANGELOG.md). When migrating to v4, use the newest version. Gradual updates will take more time and won't ease this process.
+
+## Breaking API changes
+
+- `AutoFields` component in all themes now renders a `React.Fragment` instead of a `div`. Explicitly render a wrapper component around if you need one.
+- Dropped support for `initialCount` in bridges and `ListField`s. Pass a model object to the form with the appropriate amount of initial items instead.
+- Removed the `autoField` prop from `QuickForm`, `AutoForm`, and `AutoFields` components in all themes. Use `AutoField.componentDetectorContext.Provider` instead.
+- `componentDetector` in `AutoField` components now always takes precedence over `component` property on a schema. This may make your `AutoField` render a different component when you were using both previously. If that's the case, move your schema's `component` definition to a `AutoField.componentDetectorContext.Provider` instead.
+- The constructors for all our bridges now accept an object (e.g., `{schema, validator}`) instead of individual parameters. This applies to `SimpleSchema2Bridge`, `JSONSchemaBridge`, and `ZodBridge`. Please update your constructor calls accordingly.
+- `getInitialValue` returns empty array or object instead of `undefined` for `ListField` and `NestField`.
+- `Bridge.getProps` method accepts only `name` argument now. Additional arguments are no longer supported.
+- Replaced `allowedValues` with `options` prop
+- AntD theme uses `v5` of `antd` package. Update your project to use `v5` of `antd` package.
+- MUI theme uses `v6` of `@mui/material` package. Update your project to use `v6` of `@mui/material` package.
+- Initial render doesn't trigger individual field `onChange` functions anymore. [#1343](https://github.com/vazco/uniforms/pull/1343)
+
+## Deprecated packages
+
+The following packages are deprecated and are no longer supported in v4.0.0.
+
+- Deprecate `uniforms-bootstrap3` package
+- Deprecate `uniforms-material` package
+- Deprecate `uniforms-bridge-simple-schema` package
+- Deprecate `uniforms-bridge-graphql` package
+
+If you want to continue using these packages, you can use the following commands to download the latest version of the package:
+
+```
+npx gitget https://github.com/vazco/uniforms/tree/v3.10.2/packages/<uniforms-package-name>
+```