add justfile with recommended/suggest build aliases

[maxandersen]

if you have just (https://just.systems/) installed this make for easy run of the recommend various mvn variants.

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[gsmet]

Improved the doc a bit and ignored it for CI.

[gastaldi]

That's an excellent idea. Now I can do just build instead of browsing my bash history

Here is how to enable auto-completion: https://just.systems/man/en/shell-completion-scripts.html

[gsmet]

I would use the build-fast version if I were you. I wonder if we should make it just build and drop build-fast.

[gastaldi]

I wonder why build-fast doesn't use -Dquickly too?

[gastaldi]

Out of curiosity: Why -T0.8C and not -T1C?