RFR: 8373117: Update build doc link in README.md

[Albert Mingkun Yang]

On Mon, 8 Dec 2025 15:25:01 GMT, Erik Joelsson <erikj at openjdk.org> wrote:

> The main README.md in the JDK repository has a link to https://openjdk.org/groups/build/doc/building.html. This page is being retired in favor of the markdown file in GitHub, so we should update the link to point directly to https://git.openjdk.org/jdk/blob/master/doc/building.md.

Marked as reviewed by ayang (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/28701#pullrequestreview-3554123642