* add: Tether icon
- added tether icon to icons folder in svg 24x24
- svg icon optimized with SVGOMG Online Tool
- tether icon description added to simple-icons.json with its meta
* fix: correct tether syntax in _data/simple-icons.json
* fix: correct tether syntax in _data/simple-icons.json
* fix: remove aliases and left only one
* fix: remove comma in alias
* add: guidelines for tether
