Manual documentation improvement

One strong pain point that wasn't mentioned: For rendering to do anything at all, you need to have the Render project selected.

Clicking the button does nothing and no error is shown.

Fixed (in order of priority) I'd recommend:

1. Update the documentation.
2. Show an error if no render project is selected and the user tries to render the scene.
3. Try to find the render project if there is single one (active).