Target ids are sanitised, so may not align with e.g. references in `ref` roles

[mbercx]

Describe the bug

Underscores _ inside a target header seem to get converted into hyphens -, but not for the references. E.g. this .rst snippet:

.. _test_snippet:

.. code-block:: Python

    # This is a test snippet.

Dude, check out that :ref:`test snippet <test_snippet>`!

Gets converted into:

(test-snippet)=

```{code-block} Python

# This is a test snippet.

```#

Dude, check out that {ref}`test snippet <test_snippet>`!

Expected behavior

Either the underscores should be preserved, or both the target header and references should be converted into hyphens, in case this is preferred.

Environment

• Python Version [e.g. 3.7.1]: 3.8.6
• Package versions or output of jupyter-book --version:

  $ pip freeze | grep myst
  myst-nb==0.13.0a1
  myst-parser==0.14.0a2
  rst-to-myst==0.1.2
  

• Operating System: macOS Big Sur 11.3.1

[chrisjsewell]

Heya, docutils id creation is defined here: https://github.com/chrisjsewell/docutils/blob/8adab0660b2097b4f3c32cef7e5ff4cb3c72b084/docutils/docutils/nodes.py#L2196; basically every non-alphanumeric character gets converted to a -, and so actually test_snippet should still match test-snippet (although let me know if you think this is not the case)

I'm going to mark this therefore as an documentation/improvement issue, not a bug; it would be nice to align them, but its a bit tricky to achieve in the code