docs: Add course content for Day 3 (#1943)

* docs: Add course content for Day 3

* docs: update day 3 _index.md

* made mockup more mockup

* fixed image links

* added video links for sparse vectors

* added lost in PR dissection images

* added videos/notebook

* docs: update pitstop-project.md structure

* docs: add global vocabulary for sparse vectors

* pitstop setup added

* pitstop fix

* docs: update Discord link in pitstop-project.md

---------

Co-authored-by: Kirstin <kirstin.taufertshoefer@qdrant.com>
Co-authored-by: Evgeniya Sukhodolskaya <suxodolskaya97@gmail.com>

qdrant-landing/content/course/essentials/day-3/_index.md

@@ -1,9 +1,24 @@
 ---
-title: Day 3
+title: "Day 3: Hybrid Search"
 isLesson: true
-weight: 7
+weight: 4
 ---
 
 {{< date >}} Day 3 {{< /date >}}
 
-# Day 3
+# Hybrid Search
+
+Combine dense and sparse signal for precision and recall.
+
+---
+
+## Today’s path
+
+1. Sparse Vectors and Inverted Indexes
+2. Keyword‑based search demo
+3. Hybrid Search with Score Fusion
+4. Demo: Implementing a Hybrid Search System
+5. Project: Building a Hybrid Search Engine
+
+You’ll build a hybrid engine and validate gains over dense‑only or sparse‑only search.
+