Add icons

[thibautsailly]

Add the GitHub icons and icon design guidelines to the OSRD Web Sketch Library.

• [x] Pointing to
• [x] Status
• [x] Authentification
• [x] Documents
• [x] Communication
• [x] Media
• [x] Objects
• [x] Text manipulation
• [x] Data manipulation
• [x] Interface helpers

Explain choices in the design process documentation.

[thibautsailly]

438 icons 😅