[content] improve documentation guiding from 'manual json' to yeoman

[srl295]

Note: Reference https://github.com/eclipse-theia/theia/discussions/14244

1. Reading https://theia-ide.org/docs/composing_applications/ the implication is that the 'manual' JSON approach is equivalent to the generator, with the difference that you have to manually type in the files. But it seems that in reality the approaches are a little different: In my words, the 'manual' approach basically runs theia via the CLI, where the generator builds a scaffolded application which you can then modify.

Consider something at the end of that page that describes and contrasts the generator approach as a next step.

2. When jumping into the generator at https://theia-ide.org/docs/composing_applications/ and even the guide https://theia-ide.org/docs/authoring_extensions/ which seems to be a tutorial on using the generator, there's a little bit of transition needed.

Now, i've been around especially Eclipse, but also VSCode, so I get the world where "everything is an extension", as disussed in https://theia-ide.org/docs/architecture/

However, I think what's missing is something that says, "In order to build your own IDE/Tool, what you need to do is

1. start with generating an extension that adds the top level functionality you want https://theia-ide.org/docs/authoring_extensions/#project-layout
2. besides the extension, the two generated Theia applications may then be customized for your specific needs..

Assuming all of that is true. Anwyay, this is part of my evaluation, so I'm surmising how this is all supposed to work.

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Just one more note on terminology:

• the page https://theia-ide.org/docs/authoring_extensions/#project-layout is entitled Authoring Theia Extensions and says

This guide will walk you through the process of creating Theia extensions and deploying them in your Theia-based application

• but actually, isn't it a way to build your Theia-based application if it doesnt already exist?

[JonasHelming]

Also we should clarify better when to use the generator and when the Theia IDE, see https://github.com/eclipse-theia/theia/discussions/14244#discussioncomment-10848997