Improves documentation on virtual-document-sample

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Adds a ground-up explanation on how to get up and running with the virtual-document-sample example

Addresses #666 (👹). As noted there, this is quite a beginner-attractive feature but the example is not beginner-friendly enough

After writing I saw the sample-guideline and reckon it may be in conflict with 2.2: If the sample has a corresponding guide, it should link to the guide., I still feel such a more informal guide makes more sense here, if not, just give some feedback if a similar contribution is worth on vscode-docs repo

Also note I did consider simplifying the example itself instead of changing readme, but on my case, after finally understanding everything, turns out my problem was more on the unfamiliar API style and conventions, so this informal guide seemed more fitting, and keeping the full example as a reference on the usual conventions

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