Fix goldmark toc rendering

Fix goldmark toc rendering

Previously gordmark-based TOC renderes only `KindText` and `KindString`

This commit expands target node with Goldmark's renderer

I am not sure of what are expected results as TOC contents in some (rare) cases
but Blackfriday's behaviours are fundamentally respected.

For example,
- image `[image text](link)` is rendered as `<img>` tag
- GFM AutoLink `gohugo.io` is rendered as text

* Render AutoLink as <a> tag as Blackfriday does

Fixes #6736
Fixes #6809