Updated the "What is this?" section of the Theia IDE README

[dannaf]

What it does

This PR's primary purpose is to suggest and/or clarify Theia IDE's purpose as more than merely a 'template' for developing custom applications based on the Theia platform, but also — indeed/perhaps primarily — as a 'minimum viable product' that can be used out-of-the-box. (Which should be supported by the development team as such.)

This PR thus seeks to resolve eclipse-theia/theia-website#524, particularly the last paragraph in eclipse-theia/theia-website#524 (comment), where I

suggest[ed] that the secondary, indirect project goal of supporting end-users should be upgraded slightly, albeit not too much, to formally see it as one of the primary, direct project goals to provide a working, supported, demonstrated and easy-to-get-started-with end-to-end sample/example 'out-of-the-box' browser-based IDE

___

Along the way this commit reorganizes and improves this entire paragraph/section. Particularly:

This commit also added a missing hyperlink to the Theia.cloud demo.

This commit removed the first sentence ("The Eclipse IDE is a modern and open IDE for cloud and desktop"), as in my opinion it may only add further confusion to a new user and the focus here in the README of the Theia IDE should be a clear communication of the purpose of this product (not the Eclipse IDE), and of the distinction and relation between Theia IDE and the Theia platform. (If it is truly important here to mention the Eclipse IDE I suggest that this sentence should be written more clearly, and also relocated e.g. to the end of the paragraph, rather than being the very first sentence.)

This commit relaxed the excessive '-based products based on' language.

How to test

The main task is to thoroughly consider, and discuss as needed, the high-level project's goals: I suggest that the Eclipse Theia project's goals should include — and arguably already do include, in truth — the full support of Theia IDE as a product (albeit a minimal product) for immediate use by users, as opposed to the Theia platform being a platform for development by developers.

Another task is to check that the correct Theia.cloud hyperlink was added. (Presumably so, since I just took it from the Theia website.) [And if this PR gets stuck in discussion, please consider meanwhile adding the Theia.cloud hyperlink in a separate PR, as there is really no reason it should not be here on the README in this paragraph when it is mentioned, in addition to on the website.]

Another task is to review generally the revised language of this section.

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines

[JonasHelming]

LGTM, thank you, I have two comments. We also have a conflict, as I added a link to the Docker section before I saw your PR.

[JonasHelming]

Suggested change

The Eclipse Theia IDE is a **_minimum viable product_** based on the Eclipse [Theia platform](https://theia-ide.org), which can be used out-of-the-box either on desktop or in the browser. You can [download the desktop application](https://theia-ide.org//#theiaidedownload) or [build the browser application](https://github.com/eclipse-theia/theia-blueprint#docker-build). You can also [try the latest version of the Theia IDE online](https://try.theia-cloud.io/) via Theia.cloud for up to 30 minutes per session.

The Eclipse Theia IDE is a **_minimum viable product_** based on the Eclipse [Theia platform](https://theia-ide.org), which can be used out-of-the-box either on desktop or in the browser. You can [download the desktop application](https://theia-ide.org//#theiaidedownload) or [build/use the browser application](https://github.com/eclipse-theia/theia-blueprint#docker-build). You can also [try the latest version of the Theia IDE online](https://try.theia-cloud.io/) via Theia.cloud for up to 30 minutes per session.

[JonasHelming]

I am not sure about the term "minimum viable product" which is by definition "minimal" and that is not really how we select features here. It is rather "vanilla", "default" or something like this, but I do not have a good term tbh.