chore(docs): add documentation for build --deploy-url

[changLiuUNSW]

This doc is for #3285

[merwok]

IMHO the documentation added here is very scarce; I had to read the whole original pull request to understand the intended behaviour, relationship to webpack options and interactions with base-href (this last point still unclear).

[filipesilva]

Seems to have already been added meanwhile.

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