RFR: 8273497: Building.md should link to testing md file rather than html

[Aleksey Shipilev]

On Wed, 8 Sep 2021 15:57:58 GMT, Erik Joelsson <erikj at openjdk.org> wrote:

> I don't think just changing the link here is the correct action. 

+1. Let's not do this change yet.

>     1. Create two links and mark them with .md and .html so that the reader may pick the one that makes sense depending on context.
>     2. Get rid of the html files since Github now naturally uses .md files rather than .html files for online reading.
>     3. Do nothing and continue to consider the html files as the official online documentation files.
> 
> I'm thinking 2 may be the preferred action, but would like more input from others. 

(2) looks appealing for me, for a different reason: if you regenerate `.md` -> `.html`, and choose the unexpected pandoc version (for example one provided by distro), then `.html` diff would have a lot of fluff not related to the actual change. And that would keep happening as people regenerate `.html` with their own versions of pandocs :) Removing `.html` from repo resolves this problem at its core.

We can still rewire `make update-build-docs` to e.g. `make generate-build-docs` and put the resulting HTML to `build/` somewhere, so whoever deploying the HTML files on their site can still get it easily.

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PR: https://git.openjdk.java.net/jdk/pull/5417