Auto-generated header links ignore mis-anchor headers that start with a number

[choldgraf]

Describe the bug

When section headers start with a number, they are labeled in a confusing way by the auto-header anchor generator. They simply have the structure id<num>.

For example, a document like this:

## v2.0

## 2.0

Will generate anchors like:

<a class="headerlink" href="#v2-0" title="Permalink to this headline">
<a class="headerlink" href="#id1" title="Permalink to this headline">

I think users would instead expect the ID of the second element to be 2-0

Since we describe the behavior as based on the GitHub sluggify, I also quickly checked and it looks like they do support IDs that start with numbers (that's why I'm labeling this as a bug since I think it breaks from expected behavior based on that criteria).

I think the relevant code for this might be here:

https://github.com/executablebooks/mdit-py-plugins/blob/master/mdit_py_plugins/anchors/index.py#L119-L120

originally reported in: jupyterhub/kubespawner#575

Reproduce the bug

1. Set myst_heading_anchors = 2

2. Create a page like

   # Test
   ## v2.0
   
   ## 2.0

3. Render it and note the heading IDs

List your environment

No response

[choldgraf]

Just realized another problem this surfaces - if people have multiple numbered sections (e.g. in a changelog, 3.0, 3.1, 3.2) then adding a new section at the top (e.g, 4.0) will bump the id# of all subsequent labels, making links to them valid links, but not to the correct section.

[chrisjsewell]

This may be similar to #195, whereby docutils/sphinx seems to treat such references differently.
Should probably track down in the code exactly where/why it is doing this

[chrisjsewell]

I think its treating them as "auto-targets", similar to autofootnote; the only fix may be to stick a prefix onto such slugs e.g. `if target.isdigit(): target = "-" + target"
this would not be in keeping with github slug rules, but may be the only way

[choldgraf]

maybe they could just be prefixed with num- or even n- if they started with a number?