Update doc according npm workspace

[JulienVig]

• Fixes Update module installation doc #635
  Update each module's instruction now that npm workspaces has been integrated

I reorganized the documentation with 3 different types of DISCO users in mind. What they should be able to do are concentric subsets:

1. non-technical users → want to use the UI to train and create their own custom tasks without coding
2. technical users → additionally code in Node.js to train and create their own custom tasks with discojs-node and discojs-server
3. contributors → additionally need to be able to build, test, modify and learn in-depth information

As such, I organized the doc by following these successive levels:

1. README.md for everyone and especially non-technical users
2. DEV.md for technical users and contributors, it now contains instructions for the different ways to use DISCO
3. CONTRIBUTING.md for contributors, with build, test, coding conventions and in-depth information needed to modify the code base.

Additionally:

• Removed redundant module readme (discojs, disco-core/node/web, web-client) as instructions are now centralized in DEV.md and CONTRIBUTING.md
• Merged ONBOARDING.md into CONTRIBUTING.md and deleted the former. I didn't find the difference between the two files self-explanatory enough and contributors have to go through both of them anyway so I merged the two into one file.
• Created a new example for adding a custom task to the server in docs/node_example, which I renamed docs/examples with the objective of adding further examples in the future.

Out of scope: #639 Adding custom tasks is currently very convoluted and needs a rework to make it easier for external users. Information on how to add custom tasks is not clear and not accessible enough for any of the three user categories I mentioned.

• Also fixes The CLI isn't working #609
  I expect it got fixed by the previous PRs.

[tharvik]

superbe, thanks for the tiding up 🥇

comments are not blocking, rather "nice to have"

[tharvik]

starting a server is in the server package only, but indeed discojs-node can help build a server

[tharvik]

hum, sometimes commands are from the root folder, sometimes from the local dir. we need to keep to only one style of command.

[JulienVig]

In this case I'm giving two alternative ways of starting the web-client depending on the cwd. I'm trying to always use -w but I'm sometimes wary that some readers may have missed how -w worked

[tharvik]

your call :)

[tharvik]

do we add linting support here at all? if feel a bit out of scope. the current npm run lint doesn't work (but can made to work if merge the root eslintrc with the local one)

[JulienVig]

I think that would be nice, how can I do it? I'm not what you mean by "merge tje root eslintrc"

[tharvik]

eslint uses a cascading config organisation, meaning that it aggregates all eslintrc files start from cwd and bubbling up the parent chain.
in the current case, it takes docs/examples/.eslintrc.json & .eslintrc.js, making the example non-local. you can copy the one from root in docs/examples and change its env to use node instead of mocha.