feat!: restructure erstie guide, expand

Expanded the erstie guide massively (added guide for each semester),
restructured to new URLs, added summaries details

summaries/semester4.md

@@ -1 +1,14 @@
 # Semester 4
+## Probability & Statistics
+My first Cheat-Sheet with my new helpers and a much more suitable style.
+- [Cheat-Sheet (DE)](./semester4/ps/ps-jh/probability-and-statistics-cheatsheet.pdf) Author: Janis Hutz
+- [Cheat-Sheet (DE)](./semester4/ps/ps-rb/main.pdf) Author: Robin Bacher
+
+## Formal Methods and Functional Programming
+Summarizing everything of the course, apart from Haskell.
+It also features a tips and tricks section at the end
+- [Summary (EN)](./semester4/fmfp/formal-methods-functional-programming-summary.pdf) Author: Janis Hutz
+
+## Data Modelling and Databases
+A complete script for the course
+- [Summary / Script (EN)](./semester4/dmdb/data-modelling-databases-summary.pdf) Author: Janis Hutz