add Example 2.25 from CIE4130 Lecture Notes

[rlanzafame]

Git Process:

• create branch
• do stuff
• create Pull Request and tag me for review. then when i approve the git process and high level math/code, Oswaldo can look at the content

Book: Produce 2 files and add them to the book:

• ./book_development/book/copula/example_2_25_math.md --> math, including expansion of the steps as discussed with Osawaldo
• ./book_development/book/copula/example_2_25_code.ipnyb --> implement the copula in code and make a plot. use bivariate stuff if possible