docs: Document missing SDK features across concept pages (#852)

## Summary

- Document 13 previously undocumented SDK features across 8 concept
pages with 8 new runnable code examples
- Replace all relative markdown API reference links with `<ApiLink>`
component across all concept pages
- Improve `ApifyRequestList` docstring in source code
- Add closing notes with links to platform docs on relevant pages

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: vdusek

docs/02_concepts/01_actor_lifecycle.mdx

@@ -43,7 +43,15 @@ When the Actor exits, either normally or due to an exception, the SDK performs a
     </TabItem>
 </Tabs>
 
-You can also create an [`Actor`](https://docs.apify.com/sdk/python/reference/class/Actor) instance directly. This does not change its capabilities but allows you to specify optional parameters during initialization, such as disabling automatic `sys.exit()` calls or customizing timeouts. The choice between using a context manager or manual initialization depends on how much control you require over the Actor's startup and shutdown sequence.
+You can also create an [`Actor`](https://docs.apify.com/sdk/python/reference/class/Actor) instance directly. This does not change its capabilities but allows you to specify optional parameters during initialization. The key parameters are:
+
+- `configuration` — a custom [`Configuration`](https://docs.apify.com/sdk/python/reference/class/Configuration) instance to control storage paths, API URLs, and other settings.
+- `configure_logging` — whether to set up default logging configuration (default `True`). Set to `False` if you configure logging yourself.
+- `exit_process` — whether the Actor calls `sys.exit()` when the context manager exits. Defaults to `True`, except in IPython, Pytest, and Scrapy environments.
+- `event_listeners_timeout` — maximum time to wait for Actor event listeners to complete before exiting.
+- `cleanup_timeout` — maximum time to wait for cleanup tasks to finish (default 30 seconds).
+
+The choice between using a context manager or manual initialization depends on how much control you require over the Actor's startup and shutdown sequence.
 
 <Tabs groupId="request_queue">
     <TabItem value="actor_instance_with_context_manager" label="Actor instance with context manager" default>

docs/02_concepts/02_actor_input.mdx

@@ -7,13 +7,31 @@ description: Read and validate input data passed to your Actor at runtime.
 import RunnableCodeBlock from '@site/src/components/RunnableCodeBlock';
 
 import InputExample from '!!raw-loader!roa-loader!./code/02_input.py';
+import RequestListExample from '!!raw-loader!roa-loader!./code/02_request_list.py';
+import ApiLink from '@site/src/components/ApiLink';
 
 The Actor gets its [input](https://docs.apify.com/platform/actors/running/input) from the input record in its default [key-value store](https://docs.apify.com/platform/storage/key-value-store).
 
-To access it, instead of reading the record manually, you can use the [`Actor.get_input`](../../reference/class/Actor#get_input) convenience method. It will get the input record key from the Actor configuration, read the record from the default key-value store,and decrypt any [secret input fields](https://docs.apify.com/platform/actors/development/secret-input).
+To access it, instead of reading the record manually, you can use the <ApiLink to="class/Actor#get_input">`Actor.get_input`</ApiLink> convenience method. It will get the input record key from the Actor configuration, read the record from the default key-value store,and decrypt any [secret input fields](https://docs.apify.com/platform/actors/development/secret-input).
 
 For example, if an Actor received a JSON input with two fields, `{ "firstNumber": 1, "secondNumber": 2 }`, this is how you might process it:
 
 <RunnableCodeBlock className="language-python" language="python">
     {InputExample}
 </RunnableCodeBlock>
+
+## Loading URLs from Actor input
+
+Actors commonly receive a list of URLs to process via their input. The <ApiLink to="class/ApifyRequestList">`ApifyRequestList`</ApiLink> class (from `apify.request_loaders`) can parse the standard Apify input format for URL sources. It supports both direct URL objects (`{"url": "https://example.com"}`) and remote URL lists (`{"requestsFromUrl": "https://example.com/urls.txt"}`), where the remote file contains one URL per line.
+
+<RunnableCodeBlock className="language-python" language="python">
+    {RequestListExample}
+</RunnableCodeBlock>
+
+## Secret input fields
+
+The Apify platform supports [secret input fields](https://docs.apify.com/platform/actors/development/secret-input) that are encrypted before being stored. When you mark an input field as `"isSecret": true` in your Actor's [input schema](https://docs.apify.com/platform/actors/development/input-schema), the platform encrypts the value with the Actor's public key.
+
+No special handling is needed in your code — when you call <ApiLink to="class/Actor#get_input">`Actor.get_input`</ApiLink>, encrypted fields are automatically decrypted using the Actor's private key, which is provided by the platform via environment variables. You receive the plaintext values directly.
+
+For more details on Actor input and how to define input schemas, see the [Actor input](https://docs.apify.com/platform/actors/running/input) and [input schema](https://docs.apify.com/platform/actors/development/input-schema) documentation on the Apify platform.