Running webpack-dev-server is not enough, so add a helper which takes are of
doing the needful.

* doc: Add webpack-dev-server steps

Based on work authored by Shuai Li <sli@atlassian.com>, Daniel Ornelas
<dornelas@atlassian.com>, and Lyubo Marinov <lmarinov@atlassian.com>.

Inspired by the document at jitsi/jitsi.

Executes npm update before making, in order to update latest version of packages like lib-jitsi-meet which are updated from git. The npm install method is supposed to only resolve dependencies and not update to latest versions.

Addressing readability concerns raised by @fippo

Clarify that the detailed instructions are for a 'manual' installation.