Document adding input actions in the manual, in addition to in Getting Started

**Your Godot version:**
4.4
**Issue description:**

Creating input actions is already documented in a step-by-step way in two places:
https://docs.godotengine.org/en/stable/getting_started/first_2d_game/03.coding_the_player.html#coding-the-player
https://docs.godotengine.org/en/stable/getting_started/first_3d_game/02.player_input.html#creating-input-actions

However, the [Using InputEvent](https://docs.godotengine.org/en/stable/tutorials/inputs/inputevent.html#actions) page has only a cursory mention of adding new input actions. There is a screenshot of the Project Settings > InputMap UI in [Input examples](https://docs.godotengine.org/en/stable/tutorials/inputs/input_examples.html#inputmap), too, but no deep explanation.

The manual itself should also document how to add input actions in a little more detail. This section should also be linked to from some other pages in the manual that use input actions.

The manual could link to the more detailed examples in the Getting Started tutorials, but that feels like bad practice.

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/tutorials/inputs/inputevent.html#actions
https://docs.godotengine.org/en/stable/tutorials/inputs/input_examples.html#inputmap