RFR: 8273497: building.md should link to both md and html

[Magnus Ihse Bursie]

The building.md (and the generated building.html) should link to both the markdown and the html version of the test documentation.

This is a replacement PR for https://git.openjdk.java.net/jdk/pull/5417, since I'm taking over the bug.

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Commit messages:
 - 8273497: building.md should link to both md and html

Changes: https://git.openjdk.java.net/jdk/pull/5451/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5451&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273497
  Stats: 5 lines in 2 files changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5451.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5451/head:pull/5451

PR: https://git.openjdk.java.net/jdk/pull/5451