RFR: Improve and modernize the makefile

[Magnus Ihse Bursie]

On Tue, 16 Mar 2021 13:25:28 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

>> I don't mind this change but I think it may be slightly premature as the chapters aren't nearly as stable as one might think :-)
>> The GIT rewrite will change things a lot. I would prefer to hold this change until that's done.
>
> Actually, I thought of this as a pre-requirement for the git rewrite... I started writing but just got lost in this huge index.md file.

Git is excellent at tracking file renames and merges/splits, so let's not fear future possible refactorings as a blocker for building a proper foundation.

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PR: https://git.openjdk.java.net/guide/pull/46