docs: improve Mintlify documentation quality and consistency

- Fix logo/favicon paths (build/ → assets/) and copy icons to docs/assets/
- Add page-level metadata (description, icon) to all 19 documentation pages
- Fix broken numbered list formatting in about/overview.md
- Embed YouTube walkthrough videos in index.md
- Standardize terminology: Agent (Maestro workspace), Provider (Claude/Codex/OpenCode), Session/Tab (conversation)
- Update Key Concepts table with clear definitions
- Redistribute screenshots to relevant pages (git, keyboard shortcuts, general usage)
- Transform screenshots.md into Themes Gallery page
- Add git diff/logs screenshots to git-worktrees.md
- Add command palette screenshot to keyboard-shortcuts.md
- Add session management screenshot to general-usage.md

Claude ID: ad65a8b7-fc6f-4ebc-86be-ec86f66a0120
Maestro ID: b9bc0d08-5be2-4fdf-93cd-5618a8d53b35

docs/docs.json

@@ -41,7 +41,8 @@
               "about/overview",
               "features",
               "screenshots"
-            ]
+            ],
+            "icon": "house"
           },
           {
             "group": "Getting Started",