Always show mod variables Data Panel tab

[twschiller]

Context

• https://www.notion.so/pixiebrix/Make-Mod-Variables-data-panel-Tab-less-confusing-c3456642622943c7bc9e804511e564d9?pvs=4
• The current behavior of the Page State data panel page is confusing and annoying. It only shows on starter bricks and bricks that are page-state aware. So if you're configuring/debugging a brick that uses @mod in its inputs and want explore all the current mod variables, you have to switch bricks
• Update the documentation link to point to the basic mod variable usage documentation: https://docs.pixiebrix.com/developing-mods/developer-concepts/data-context/using-mod-variables

Definition of Done

• Always show mod variables Data Panel tab
• Rename tab from "Page State" to "Mod Variables"