[docs] Usage Guide webpage should address the issue of updating and maintaining projects, created with vue-cli

[revelt]

What problem does this feature solve?

Our competition, create-react-app, has extensive documentation of how to update both their CLI and apps which were created by their CLI. We should do the same. Currently maintenance aspect is missing in vue-cli Guide documentation pages.

Our vue-cli Guide webpage should address the challenge of maintaining an app, created with vue-cli:

• how to update dependencies in the project, bootstrapped with vue-CLI (does user have to run CLI which will take care of updates or update all dependencies reinstalling them or what)
• how to update the vue-CLI itself (confirm is it as simple as global install via npm)

What does the proposed API look like?

we should aim to document all practical aspects of maintaining a project which was created by vue-cli

[LinusBorg]

We should indeed improve docs here.

The main story is pretty simple:

Update all your cli dependencies like any other npm dependencies. If anything breaks, do the usual schtick: remove node_modules and reinstall everything to get dependencies cleared, then check for any problems with your own config and the new version of plugins.

But some guidance would be surely helpful nonetheless.

Sidenote: we will get a vue update command, see #2428