docs: add upgrading guide to v03 (#464)

### Description

- Add upgrading guide from v0.2.x to 0.3.0.
- There were 2 breaking changes:
  - declaration of public & private interface,
  - request queue v2.

### Issues

- N/A

### Testing

- Website was rendered locally.

### Checklist

- [x] CI passed

---------

Co-authored-by: Jan Buchar <[email protected]>

docs/upgrading/upgrading_to_v03.md

@@ -0,0 +1,38 @@
+---
+id: upgrading-to-v03
+title: Upgrading to v0.3
+---
+
+This page summarizes most of the breaking changes between Crawlee for Python v0.2.x and v0.3.0.
+
+## Public and private interface declaration
+
+In previous versions, the majority of the package was fully public, including many elements intended for internal use only. With the release of v0.3, we have clearly defined the public and private interface of the package. As a result, some imports have been updated (see below). If you are importing something now designated as private, we recommend reconsidering its use or discussing your use case with us in the discussions/issues.
+
+Here is a list of the updated public imports:
+
+```diff
+- from crawlee.enqueue_strategy import EnqueueStrategy
++ from crawlee import EnqueueStrategy
+```
+
+```diff
+- from crawlee.models import Request
++ from crawlee import Request
+```
+
+```diff
+- from crawlee.basic_crawler import Router
++ from crawlee.router import Router
+```
+
+## Request queue
+
+There were internal changes that should not affect the intended usage:
+
+- The unused `BaseRequestQueueClient.list_requests()` method was removed
+- `RequestQueue` internals were updated to match the "Request Queue V2" implementation in Crawlee for JS
+
+## Service container
+
+A new module, `crawlee.service_container`, was added to allow management of "global instances" - currently it contains `Configuration`, `EventManager` and `BaseStorageClient`. The module also replaces the `StorageClientManager` static class. It is likely that its interface will change in the future. If your use case requires working with it, please get in touch - we'll be glad to hear any feedback.

website/sidebars.js

@@ -103,22 +103,22 @@ module.exports = {
         //         },
         //     ],
         // },
-        // {
-        //     type: 'category',
-        //     label: 'Upgrading',
-        //     link: {
-        //         type: 'generated-index',
-        //         title: 'Upgrading',
-        //         slug: '/upgrading',
-        //         keywords: ['upgrading'],
-        //     },
-        //     items: [
-        //         {
-        //             type: 'autogenerated',
-        //             dirName: 'upgrading',
-        //         },
-        //     ],
-        // },
+        {
+            type: 'category',
+            label: 'Upgrading',
+            link: {
+                type: 'generated-index',
+                title: 'Upgrading',
+                slug: '/upgrading',
+                keywords: ['upgrading'],
+            },
+            items: [
+                {
+                    type: 'autogenerated',
+                    dirName: 'upgrading',
+                },
+            ],
+        },
         {
             type: 'doc',
             label: 'Changelog',