Add mdBook/GitBook/MkDocs icons

[Fexiven]

The icon I want is:

• Name: mdBook, GitBook, MkDocs
• URL: https://github.com/rust-lang/mdBook
• URL: https://www.gitbook.com/
• URL: https://www.mkdocs.org/
• Link to icon: mdBook SVG & PNG
• Link to icon: GitBook Assets
• Link to icon: MkDocs PNG

• Vector version of the logo: see above
• Official style guide or brand guide: see above

[Vidhi154]

sir, can i take this issue ?