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

[Aleksey Shipilev]

On Thu, 9 Sep 2021 18:51:10 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

> 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.

Marked as reviewed by shade (Reviewer).

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