Use `yarn` instead of `npm` in docs and scripts (#796)

* Use `yarn` instead of `npm` in docs and scripts

Signed-off-by: Andrew Ferrazzutti <andrewf@element.io>

* Enforce node and yarn versions

Signed-off-by: Andrew Ferrazzutti <andrewf@element.io>

* Give up on trying to auto-enforce yarn 1.x

Instead, just document how to install it

Signed-off-by: Andrew Ferrazzutti <andrewf@element.io>

* Changelog: Clarify that Yarn is required

Signed-off-by: Andrew Ferrazzutti <andrewf@element.io>