Adds a cheatsheet for Java fixes issue #42

[Tristan-Raz]

This adds Python and Java equivalent functions, operators, and one-liners to show how different operators are. This should help the students with Python knowledge.

[bnmnetp]

This seems odd to include a section AFTER the backmatter. This seems like an appendix to me and should be xi:included inside the backmatter. Therefore it should also have an appendix tag instead of a section

PreTeXt tries to help you semantically structure your book. Its worth following its lead!

[Tristan-Raz]

Thank you for your help! I've now made the change, converting the section to an appendix and including it inside the backmatter tag. I will make sure to follow PreTeXt's lead on structure in the future.

[pearcej]

This leaves the repo in a broken state. Maybe you have some uncommitted changes?

1. The closing tag of the index needs to be replaced. Maybe you accidentally deleted it?
2. You need to change main.ptx to include the new appendix where it should go.

[pearcej]

This is better, but it needs a full checkover. Here are some issues I see immediately:

1. Floor Division //: This operator does not exist in Java. Java uses regular / for division, and for integer division, you'd use / with integer operands or Math.floorDiv().
2. Exponent **: This operator does not exist in Java. Java uses Math.pow() for exponentiation.
3. Logical operators and, or, not: These are Python operators, not Java. Java uses &&, ||, and !.

There is a LOT here! Often, less is better than more. I recommend we only include things that are discussed in this text, which is supposed to be a simple transition. Do we really want bitwise operations, for example?

[pearcej]

This is much improved! Let's make a few more changes:

1. Please use c tags around all code snippets and commands regardless of language.
2. Please organize the bullets in some less random fashion. Maybe put similar things together?
3. Please give examples for each of the bullets since only some have them.