Added examples of f-string usage in 'Strings-and-Regular-Expressions.ipynb'

[akhaund]

Added examples of f-string usage to string formatting section.
1.
f"The value of pi is {pi}"

first, last = 'A', 'Z'
f"""First letter: {first}. Last letter: {last}."""

f"pi = {pi:.3f}"

print(f"{pi=}")