Add links to other walkdir items in DirEntry docs

[KodrAus]

Relevant API Guideline

Add reference links in the DirEntry docs prose when mentioning other walkdir items:

• [ ] filter_entry in root skip hidden files and directories efficiently on unix example
• [ ] path, file_name and follow_links in DirEntry
• [ ] WalkDir::new in DirEntry::path
• [ ] follow_links in DirEntry::path_is_symbolic
• [ ] follow_links in DirEntry::metadata
• [ ] follow_links in DirEntry::file_type