[PS] Start notes & remarks update

semester4/ps/ps-jh/TODO.md

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+# TODOs
+- [ ] Add many more notes
+- [ ] Possibly: restrucutre to group definitions, then put theorems / remarks separately
+- [ ] Tips & Tricks
+- [ ] Better examples
+- [ ] Shorten defs, etc
+- [ ] Link together (decide on style for that)