Tidy up `Right-click Menu` section in `Viewer` tutorial

[DragaDoncila]

🧰 Task

• Update heading: Currently we have pretty good docs for everything that you can do when right-clicking on a layer, but the heading for the menu is just Right-click Menu which isn't very informative. We should update the heading to something like Layer contextual menu or Layer actions
• Fix Merge RGB subheading: There is a rendering issue with the Merge RGB heading where it appears italicised but it should be bolded
• Update actions: Make sure all actions that can show up in the contextual menu are documented. Check this by opening different layer types and right-clicking on the layer. Also check what happens when you have multiple layers linked.