Include logos in docs

[Chris-Johnston]

This PR incorporates the new logos into the docs.

• Adds the purple logo to the navbar. Because the logo that docfx supplies is 38pt x 38pt, I added a CSS rule that restricts the image to that size, with some padding.
• Adds the logo to the index page. Supports changing colors based on the theme using the css background property. The src attribute of the img tag is actually left empty. (No JS required, either)
• Adds a favicon.ico

Image of incorrect alt tag. I've edited the <title> attribute of the SVG to read "Discord.Net Docs" as a workaround. Also, I think by coincidence, the built-in :hover style for the logo just happens to work great.