feat: add substring generator one-liner to python_oneliners.md

@@ -203,3 +203,9 @@ merged = {item['id']: item for item in list_of_dicts}

import math; fact = math.factorial(n)

```

**Example:** `n=5 → 120`

## Find all substrings of a string

```python

[s[i:j] for i in range(len(s)) for j in range(i + 1, len(s) + 1)]

```

**Example:** `"abc" → ["a", "ab", "abc", "b", "bc", "c"]`