Add serve-docs command to mach

This enables the user to run `./mach serve-docs` (with an optional port
number), starting a local web server that hosts the documentation for
Servo and Rust.

This closes #3807
auto merge of #3907 : Munksgaard/servo/serve-docs, r=jdm

This enables the user to run `./mach serve-docs` (with an optional port
number), starting a local web server that hosts the documentation for
Servo and Rust.

This closes #3807