[jdk17u-dev] RFR: 8273497: building.md should link to both md and html

[Ben Taylor]

Documentation-only fix.
This patch and [JDK-8285093](https://bugs.openjdk.org/browse/JDK-8285093) are needed to make [JDK-8293550](https://bugs.openjdk.org/browse/JDK-8293550) clean.

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Commit messages:
 - Backport 5bfd043e16fa79a101a1bee38c59d42d4cd69a48

Changes: https://git.openjdk.org/jdk17u-dev/pull/914/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=914&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8273497
  Stats: 5 lines in 2 files changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/914.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev pull/914/head:pull/914

PR: https://git.openjdk.org/jdk17u-dev/pull/914