Enable markdownlint no-duplicate-header (#27500)

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

.markdownlint.yaml

@@ -6,7 +6,6 @@ line-length: {code_blocks: false, tables: false, stern: true, line_length: -1}
 no-alt-text: false
 no-bare-urls: false
 no-blanks-blockquote: false
-no-duplicate-header: {allow_different_nesting: true}
 no-emphasis-as-header: false
 no-empty-links: false
 no-hard-tabs: {code_blocks: false}