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GH-45541: [Doc][C++] Render ASCII art as-is (#45542)

### Rationale for this change

[union c_type]https://arrow.apache.org/docs/cpp/api/datatype.html#_CPPv4N5arrow14BinaryViewType6c_typeE in the BinaryViewType class describes data layout using ASCII art. It rendered a non-readable layout.

```
- Entirely inlined string data |-—|———–—| ^ ^ | | size in-line string data, zero padded

- Reference into a buffer |-—|-—|-—|-—| ^ ^ ^ ^ | | | | size | | `——. prefix | | buffer index | offset in buffer
```

It would be better render ascii as is.

```
 - Entirely inlined string data

  |----|--------------|
   ^    ^
   |    |
size    in-line string data, zero padded

- Reference into a buffer

  |----|----|----|----|
   ^    ^    ^    ^
   |    |    |    |
size    |    |    `------.
    prefix   |           |
          buffer index   |
                    offset in buffer
```

### What changes are included in this PR?

Render the ASCII part as-is.

### Are these changes tested?

No.

### Are there any user-facing changes?

No.
* GitHub Issue: #45541

Authored-by: Hiroyuki Sato <hiroysato@gmail.com>
Signed-off-by: Bryce Mecum <petridish@gmail.com>