Backport #27500 by @silverwind
Duplicate headers in a single Markdown document are problemlematic
because the auto-generated links won't be stable. Enable this rule with
no exceptions which is also the default of `markdownlint`. For example:
```md
# A
## Example
# B
## Example
```
Docasaurus will generated `example` and `example-1` links for this. If
the first heading is altered, the link `example` will unexpectedly move
to the second example heading.
Ref: https://github.com/go-gitea/gitea/pull/27461#discussion_r1347987659
Co-authored-by: silverwind <me@silverwind.io>
