docs: add v0.6.0 quests/achievements demo video + refresh site video grid

Wire the demo video (makertube uSGJ2MHEFjSSKxMiJBJ6Y5) into quests.md,
achievements.md, content-authoring.md and admin-plugin.md, and feature it in
the landing page's video grid (replacing the oldest embed; that video stays
linked from docs/inventory.md).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/achievements.md

@@ -1,5 +1,7 @@
 # Achievements
 
+> 📹 **Demo:** [quests, achievements & the no-code editors in action](https://makertube.net/w/uSGJ2MHEFjSSKxMiJBJ6Y5)
+
 The achievement system ([`src/systems/Achievements.luau`](../src/systems/Achievements.luau)) tracks
 **milestones**: always-on counters that unlock a badge (once) when they cross a threshold — with a
 toast and an Achievements menu tab. The architecture is ported from The Counter Earth's proven

docs/admin-plugin.md

@@ -16,7 +16,7 @@ Attributes in the Explorer. It adds two toolbar buttons under **SurvivorCore**:
 The rest of this page covers the Survival Stats editor; both install the same way. It's the
 [Builder / Admin plugin](https://github.com/TemujinCalidius/SurvivorCore/issues/11).
 
-> 📹 **Demos:** [HUD, survival stats & the admin plugin](https://makertube.net/w/xqX7wfRpTqd9L9BkozCS1P) · [no-code item & gatherable creation](https://makertube.net/w/mCneurjoY3Av6yi48VsGQE) · [no-code weapon, ammo & mob creation](https://makertube.net/w/tyn8JEMG3CaMbTXid8osdU)
+> 📹 **Demos:** [HUD, survival stats & the admin plugin](https://makertube.net/w/xqX7wfRpTqd9L9BkozCS1P) · [no-code item & gatherable creation](https://makertube.net/w/mCneurjoY3Av6yi48VsGQE) · [no-code weapon, ammo & mob creation](https://makertube.net/w/tyn8JEMG3CaMbTXid8osdU) · [no-code quest & achievement creation](https://makertube.net/w/uSGJ2MHEFjSSKxMiJBJ6Y5)
 
 ## Install
 

docs/content-authoring.md

@@ -1,6 +1,6 @@
 # No-code content authoring
 
-> 📹 **Demos:** [creating mobs, weapons & ammo (with full stats)](https://makertube.net/w/tyn8JEMG3CaMbTXid8osdU) · [creating an item + gatherable](https://makertube.net/w/mCneurjoY3Av6yi48VsGQE)
+> 📹 **Demos:** [creating quests & achievements](https://makertube.net/w/uSGJ2MHEFjSSKxMiJBJ6Y5) · [creating mobs, weapons & ammo](https://makertube.net/w/tyn8JEMG3CaMbTXid8osdU) · [creating an item + gatherable](https://makertube.net/w/mCneurjoY3Av6yi48VsGQE)
 
 The engine ships **zero** items, resources, or recipes — your game supplies them. You can do this
 two ways, and they coexist:

docs/quests.md

@@ -1,5 +1,7 @@
 # Quests
 
+> 📹 **Demo:** [quests, achievements & the no-code editors in action](https://makertube.net/w/uSGJ2MHEFjSSKxMiJBJ6Y5)
+
 The quest system ([`src/systems/Quests.luau`](../src/systems/Quests.luau)) gives players **goals**:
 accept a quest, work its objectives, claim the reward. Progress is driven entirely by the events
 the other systems already fire (gather / craft / kill / use), so a quest needs **zero wiring** —