Improvements to documentation (#1919)

* Typo: Github => GitHub

* Typo: windows => Windows, docker => Docker, and some punctuations

* "QMK Introduction" links to the right file

* "Unix" rather than "UNIX", which is a trademark

* Directory name is "keyboards", not "keyboard"

* "handwired" is a subdirectory of "keyboards"

* Punctuation and minor fixes

* macOS rather than Mac

* Punctuation and other minor fixes

* Vagrant Guide links to an existing file

* Jun Wako referenced with his name rather than his nickname

* Saxon genitive 's outside the link