web: add commands to start web dev-server to the root package.json

Created by: valerybugakov

Description

After some time, I found a couple of ways to speed up the web application's local development process for myself.

1. I usually start Webpack dev-server separately from all other services to be able to restart it without waiting for other services.
2. I try to run a minimal set of the backend services required to power up web application UI. By scientific experimentation (by randomly enabling and disabling services using the --only flag for start.sh script) I determined what services are necessary to load the web application without errors. After that, my laptop is happier because it has more resources to allocate to VSCode and Chrome and I'm still able to work on most of the frontend tasks.

With this PR, I want to share my findings. Also, curious to hear about your tips and tricks for local development.

Changes

• Added yarn install to the web serve commands in sg.config.yaml.
• Added Local development section to web_app.md with commands description.