Add documentation to sliding window permutation solution

[Copilot]

Documents the sliding window algorithm in 0567-permutation-in-string.py with complexity analysis and inline explanations.

Changes

• Docstring: Added algorithm overview, O(len(s1) + len(s2)) time complexity, O(1) space complexity
• Inline comments: Explained frequency array approach, matches counter optimization, and window slide mechanics
• Repository hygiene: Added __pycache__/ to .gitignore

Example

# Slide the window across s2
l = 0
for r in range(len(s1), len(s2)):
    # If all 26 characters match, we found a permutation
    if matches == 26: return True

    # Add the new character entering the window (right side)
    index = ord(s2[r]) - ord('a')
    s2Count[index] += 1
    if s1Count[index] == s2Count[index]:
        # This character now matches, increment matches
        matches += 1
    elif s1Count[index] + 1 == s2Count[index]:
        # This character was matching before, but now exceeds count
        matches -= 1

The key insight documented: tracking match count across 26 letters avoids O(26) array comparison per window position.

Original prompt

On 0567-permutation-in-string.py, Add comments explaining the solution

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