Missing copyrights for icons

[stefan6419846]

The icons in https://github.com/pradyunsg/furo/blob/main/src/furo/assets/styles/variables/_icons.scss are attributed to two different sources, but are missing their original copyright notices.

• The search symbol refers to https://tablericons.com/, where the source is available at https://github.com/tabler/tabler-icons Its license (MIT) requires including the original copyright notice inside the distribution: https://github.com/tabler/tabler-icons/blob/main/LICENSE
• The other icons refer to https://squidfunk.github.io/mkdocs-material/reference/admonitions/ I guess the source files are at https://github.com/squidfunk/mkdocs-material/tree/master/material/templates/.icons(?) They have differing icon licenses (CC-BY-4.0, Apache-2.0, MIT or CC0-1.0, depending on the third-party icon set), usually requiring proper attribution and possibly including the original notice.

As it stands, this violates the icon licenses and should be updated once it has been determined what the actual original source files are.