Fix style of the first ten solutions for Project Euler (#3242)

* Fix style of the first ten solutions for Project Euler

- Unify the header docstring, and add reference URLs to wikipedia
  or similar
- Fix docstrings to be properly multilined
- Add newlines where appropriate
- Add doctests where they were missing
- Remove doctests that test for the correct solution
- fix obvious spelling or grammar mistakes in comments and
  exception messages
- Fix line endings to be UNIX. This makes two of the files seem
  to have changed completely
- no functional changes in any of the solutions were done
  (except for the spelling fixes mentioned above)

* Fix docstrings and main function as per Style Guide

project_euler/problem_002/sol2.py

@@ -1,39 +1,46 @@
-"""
-Problem:
-Each new term in the Fibonacci sequence is generated by adding the previous two
-terms. By starting with 1 and 2, the first 10 terms will be:
-
-    1,2,3,5,8,13,21,34,55,89,..
-
-By considering the terms in the Fibonacci sequence whose values do not exceed
-n, find the sum of the even-valued terms. e.g. for n=10, we have {2,8}, sum is
-10.
-"""
-
-
-def solution(n: int = 4000000) -> int:
-    """Returns the sum of all fibonacci sequence even elements that are lower
-    or equals to n.
-
-    >>> solution(10)
-    10
-    >>> solution(15)
-    10
-    >>> solution(2)
-    2
-    >>> solution(1)
-    0
-    >>> solution(34)
-    44
-    """
-    even_fibs = []
-    a, b = 0, 1
-    while b <= n:
-        if b % 2 == 0:
-            even_fibs.append(b)
-        a, b = b, a + b
-    return sum(even_fibs)
-
-
-if __name__ == "__main__":
-    print(solution(int(input().strip())))
+"""
+Project Euler Problem 2: https://projecteuler.net/problem=2
+
+Even Fibonacci Numbers
+
+Each new term in the Fibonacci sequence is generated by adding the previous
+two terms. By starting with 1 and 2, the first 10 terms will be:
+
+1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...
+
+By considering the terms in the Fibonacci sequence whose values do not exceed
+four million, find the sum of the even-valued terms.
+
+References:
+    - https://en.wikipedia.org/wiki/Fibonacci_number
+"""
+
+
+def solution(n: int = 4000000) -> int:
+    """
+    Returns the sum of all even fibonacci sequence elements that are lower
+    or equal to n.
+
+    >>> solution(10)
+    10
+    >>> solution(15)
+    10
+    >>> solution(2)
+    2
+    >>> solution(1)
+    0
+    >>> solution(34)
+    44
+    """
+
+    even_fibs = []
+    a, b = 0, 1
+    while b <= n:
+        if b % 2 == 0:
+            even_fibs.append(b)
+        a, b = b, a + b
+    return sum(even_fibs)
+
+
+if __name__ == "__main__":
+    print(f"{solution() = }")