Rollup merge of #154812 - kaepr:fira-mono-license, r=notriddle,lolbinarycat

Update Fira Mono License Information

Update license information for Fira Mono font.

The license itself seems to be identical as `FiraSans` license, but didn't reuse it's file as it was explicitly named `FiraSans-LICENSE`.

Added a new LICENSE from https://github.com/mozilla/Fira/blob/master/LICENSE and updated `build` and `static` files setup.

`./x doc library` worked fine locally and was able to see the mono license being minified and served as well.

Author: JonathanBrouwer