Fix#9080
Example, where `[]` is a 2 byte non-breaking space:
```
def f():
""" Docstring header
^^^^ Real indentation is 4 chars
docstring body, over-indented
^^^^^^ Over-indentation is 6 - 4 = 2 chars due to this line
[] [] docstring body 2, further indented
^^^^^ We take these 4 chars/5 bytes to match the docstring ...
^^^ ... and these 2 chars/3 bytes to remove the `over_indented_size` ...
^^ ... but preserve this real indent
```
Closes https://github.com/astral-sh/ruff/issues/8695
We track the smallest offset seen for overindented lines then only
reduce the indentation of the lines that far to preserve indentation in
other lines. This rule's behavior now matches our formatter, which is
nice.
We may want to gate this with preview.