Skip generating md links in 'pre' blocks

[t-kalinowski]

Markdown does not support links in pre-formatted code blocks.

With the current release, markitdown mangles code chunks by generating links.

For example: markitdown https://r4ds.hadley.nz/base-r will generate code chunks that look like this:

```
# Left
[hist](https://rdrr.io/r/graphics/hist.html)(diamonds$carat)

# Right
[plot](https://rdrr.io/r/graphics/plot.default.html)(diamonds$carat, diamonds$price)
```

With this patch, the same code chunk is now correctly generated

```
# Left
hist(diamonds$carat)

# Right
plot(diamonds$carat, diamonds$price)
```

[afourney]

Nice find. Thanks for the PR.

We should add a test for this to avoid regressing. But it's simple enough that we can add tests in another PR.