feature: render completion progress in parent checkboxes

Is your feature request related to a problem? Please describe.

When looking at markdown TODOs I like to know that the parent task is started at the glance of an eye, and see how much subtasks I've completed.

Describe the solution you'd like

It would be awesome to render (not modify) the checkbox icon or line to describe either that some of the task's children have already been checked, or even describe how much of them have been checked.

This is how it would like raw:

- [ ] Parent task
  - [x] Child task 1
  - [ ] Child task 2

This is how it currently renders [ ]/[x] represent the single character that render-markdown uses to replace - [ ] or - [x]:

[ ] Parent task
  [x] Child task 1
  [ ] Child task 2

IDEA 1: This is how neorg does it ([-] here should be imagined as a single character clock icon that replaces - [-], already doable in the config):

[-] Parent task   [1/2] (50%)
  [x] Child task 1
  [ ] Child task 2

This means that when one toggles a child task to DONE, the rendering of the parent task should auto-update.

IDEA 2: same without the [1/2] (50%)
IDEA 3: only the [1/2] (50%), no rendering of a third checkox icon signifying an intermediary state (like a clock icon)
** IDEA 4**: only the icon, but multiple. For example the nerdfont icon nf-md-circle_slice_6 and its siblings would allow to show the parent checkbox increasing in size as the amount of children tasks are checked.

Describe alternatives you've considered

• Bullets.nvim does this but it actually writes the - in - [-] instead of just rendering a nerdfont character over it. Also bullets.nvim allows for multiple icons, so I can use a checkbox that fills progressively depending on how many children checkboxes are checked.
• Neorg.

Additional information

No response

Start by locating the checkbox rendering and nested Markdown task handling in render-markdown.nvim. First resolve which proposed representation to support, then define how parent progress updates when child tasks change without modifying the source Markdown; done should include consistent rendering for the chosen behavior.