[GHA] [DOCS] Custom actions document (#22079)

* slightly adapt wording in overview, start with runners doc

* populate table; add section about runners choosing

* wording

* use runners

* add doc about Docker images

* use better formatting for pros and cons

* fix typo

* add info about custom actions

docs/dev/ci/github_actions/custom_actions.md

@@ -1,11 +1,53 @@
 # Overview of the Custom GitHub Actions used in the OpenVINO GitHub Actions CI
 
-There are several actions written specifically for the needs of the OpenVINO workflows.
+Several actions are written specifically for the needs of the OpenVINO workflows.
 
 You can find all the custom actions and their source code [here](../../../../.github/actions).
 
+Refer to the official [custom GitHub Action documentation](https://docs.github.com/en/actions/creating-actions/about-custom-actions) for more information.
+
 ## Available Custom Actions
 
-* Setup Python
-* System Info Print
+* [Setup Python](#setup-python)
+* [System Info Print](#system-info-print)
 * Smart CI (see details: [feature documentation](./smart_ci.md))
+
+## Setup Python
+
+This custom action installs the required Python version and environment variables on the runner. 
+
+Under the hood it uses the GitHub-provided `actions/setup-python` and community-provided `deadsnakes-action` depending on the machine architecture. 
+`actions/setup-python` does not work on the Linux ARM64 machines so `deadsnakes-action` is used instead.
+
+### Usage
+```yaml
+  - name: Setup Python ${{ env.PYTHON_VERSION }}
+    uses: ./openvino/.github/actions/setup_python
+    with:
+      version: '3.11'
+      pip-cache-path: ${{ env.PIP_CACHE_PATH }}
+      should-setup-pip-paths: 'true'
+      self-hosted-runner: 'true'
+      show-cache-info: 'true'
+```
+where:
+* `version` - the Python version to install in the `MAJOR.MINOR` format
+* `pip-cache-path` - the path to the `pip` cache on the mounted share. Read more about shares and caches [here](./caches.md)
+* `should-setup-pip-paths` - whether the action should set up the `PIP_CACHE_DIR` and `PIP_INSTALL_PATH` environment variables for later usage
+* `self-hosted-runner` - whether the runner is self-hosted. Read more about available runners [here](./runners.md)
+* `show-cache-info` - whether the action should show the share space occupied by the `pip` cache
+
+## System Info Print
+
+This custom action prints the system information in the standard output:
+* Operating system
+* Machine architecture
+* CPU, RAM and memory information
+
+Works on Linux, macOS and Windows.
+
+### Usage
+```yaml
+  - name: System info
+    uses: ./openvino/.github/actions/system_info
+```