doc: clarify ICU source layout for --with-icu-source

[skdas20]

Fixes #60947

Summary

Clarifies --with-icu-source build docs to match current ICU release packaging and expected directory layout:

• Updated example archive name from icu4c-##.#-src.tgz to icu4c-##.#-sources.tgz.
• Clarified that --with-icu-source must point to an extracted ICU source archive layout.
• Added note that a plain ICU Git checkout is not sufficient unless prepared into that layout.

Validation

Ran markdown lint on the changed file:

node tools/lint-md/lint-md.mjs BUILDING.md

Result: exit code 0.

[nodejs-github-bot]

Review requested:

• @nodejs/build
• @nodejs/tsc

[Renegade334]

I'm not sure this is incredibly clear with respect to what the paths or "prepared to match" mean. Converting from the source code tree to the compiled source archive is an active build step (make dist), not just a "layout"; I don't think we need to mention what to do with a checked-out source tree, only that it's not the way to do it.

Suggested change

	The directory passed to `--with-icu-source` must contain the extracted ICU
	source layout (for example, `source/data/in/icudt*l.dat`). A plain Git checkout
	of the ICU repository is not sufficient unless it has been prepared to match
	this layout.
	(Note that simply cloning the icu4c repository will not work here; the source
	directory must have been unpacked from the compiled `-sources` archive.)

[Renegade334]

Also need to update the archive names here.

[github-actions]

This pull request has been marked as stale due to 90 days of inactivity.
It will be automatically closed in 30 days if no further activity occurs. If this is still relevant, please leave a comment or update it to keep it open.

[skdas20]

Thanks @Renegade334 — applied both. Used your wording verbatim for the icu4c note, since you're right that make dist is an active build step rather than a matter of layout, and saying only that a clone isn't the way to do it is clearer than describing what to do with one. Also updated the archive name in the Windows section to -sources.tgz to match.