[tests] tests for the parser

[deven367]

This PR adds tests for the new parser that will be used in the when we will create new documentation. The idea is that the parser reads the .md file based documentation and when it sees a code block like this

# some title

other imp info ....

::: coreforecast.differences.num_diffs

renders to

# some title

other imp info ....

### `num_diffs`

```python
num_diffs(x, max_d=1)
```

Find the optimal number of differences

**Parameters:**

Name | Type | Description | Default
---- | ---- | ----------- | -------
`x` | <code>[ndarray](#numpy.ndarray)</code> | Array with the time series. | *required*
`max_d` | <code>[int](#int)</code> | Maximum number of differences to consider. Defaults to 1. | <code>1</code>

**Returns:**

Name | Type | Description
---- | ---- | -----------
`int` | <code>[int](#int)</code> | Optimal number of differences.

It will render the documentation for the fn/class in-place in the md file

[deven367]

I've temporarily skipped the tests for datasetsforecast for now, I'm suspecting a python version/dependency issue. I can reproduce the issue in a codespace, but not locally.

Besides that the overall PR is in good shape to get some feedback and comments! :)

[JQGoh]

For the given tests, they look good to me. But I suggest that we try to resolve the dependency issue and have validations for datasets cases that you mention.