action: use setup-python@v4

action: cache pip dependencies

feat: merge only when atleast one file changes

feat: create a combined.slides.pdf too

maintain: split code of pagesplitter

fix: fix build error due to unmaintained 'dotenv'

alag pdf: Split pdf implemented

env: Use .env file when present

minor: Add () for clarity

typo: Indentation

action: Exit status = returned code from main

combined: Fix typo (added not)

combined: Skip Lecture 0 ... .pdf

action: Fix action

action: Fix action