RFR: 8247591: Document Alpine Linux build steps in OpenJDK build guide

[Aleksei Voitylov]

Please review the build guide update for Alpine Linux.

building.html was generated by "make update-build-docs". However, this command produced a slightly different
building.html, with some other sections modified as well. This is probably due to another version of the tooling
installed. I have manually removed the modifications to other, not relevant sections of the building.html change
introduced by the tool, and kept only the changes relevant to the md file change.

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Commit messages:
 - JDK-8247591: Document Alpine Linux build steps in OpenJDK build guide

Changes: https://git.openjdk.java.net/jdk/pull/512/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=512&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8247591
  Stats: 51 lines in 2 files changed: 51 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/512.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/512/head:pull/512

PR: https://git.openjdk.java.net/jdk/pull/512