Implement placeholder replacement for language-specific content

## Feature Request
Add support for replacing placeholders in base templates with language-specific content during merge.

## Use Case
When maintaining documentation for multiple programming languages, there's often a common structure with language-specific sections. We need a way to inject language-specific content into common templates.

## Proposed Solution
- Detect placeholders like `` in base templates
- Extract corresponding content from language-specific files (e.g., testing commands from Go-specific file)
- Replace the entire placeholder block with the extracted content

## Example
Base template:
```markdown
## Testing

```

Language file:
```markdown
### Testing commands
- Unit tests: `go test ./...`
- Race tests: `go test -race ./...`
```

Result: The placeholder block is replaced with the test commands.

No implementation files, tests, or entry points are named. Start by locating the merge logic for base and language-specific Markdown files, then trace how template content is parsed. Done means matching the specified placeholder blocks, extracting the corresponding language-specific content, and replacing the complete blocks during merge.

Written by the indexing model from the issue text.