Add instructions for the UMD build to the Quick start guide

[filipsobol]

Suggested merge commit message (convention)

Docs: Add instructions for the UMD build to the Quick start guide.

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[Witoso]

Please sync this with what @gorzelinski started to prepare in the cloud branch.

[godai78]

LGTM. We must keep in mind Melman sync for this, too.

[gorzelinski]

I added a few comments.

[Witoso]

@gorzelinski is this similar to what you have on your branch? I feel we are touching this from different angles, but I'm not sure.

[gorzelinski]

@gorzelinski is this similar to what you have on your branch? I feel we are touching this from different angles, but I'm not sure.

Here's a table of contents of the vanilla js cloud quick start on the epic branch. There is an "Installing CKEditor 5 from CDN" section where I describe the usage with global variables and UMD modules. The "Installing premium features from CDN" is the same but for premium features.

The "Advanced setup with import maps" section contains the setup with import maps and ES Modules (for both - open-source and premium features).

So, the sections are similar. However, nowhere in the guide do I mention "build," "UMD," or "ES modules" directly.

[gorzelinski]

LGTM