Add document with installation instructions

[zachgk]

This gives instructions for installing DJL, working with nightly snapshots, and
building from source. It can then be linked to in various documentation and help requests.

[codecov-commenter]

Codecov Report

Merging #1548 (080944a) into master (bb5073f) will decrease coverage by 1.28%.
The diff coverage is 51.05%.

@@             Coverage Diff              @@
##             master    #1548      +/-   ##
============================================
- Coverage     72.08%   70.80%   -1.29%     
- Complexity     5126     5370     +244     
============================================
  Files           473      501      +28     
  Lines         21970    23427    +1457     
  Branches       2351     2545     +194     
============================================
+ Hits          15838    16587     +749     
- Misses         4925     5565     +640     
- Partials       1207     1275      +68     

Impacted Files	Coverage Δ
api/src/main/java/ai/djl/modality/cv/Image.java	69.23% <ø> (-4.11%)	⬇️
...i/djl/modality/cv/translator/BigGANTranslator.java	21.42% <ø> (-5.24%)	⬇️
...odality/cv/translator/BigGANTranslatorFactory.java	33.33% <0.00%> (+8.33%)	⬆️
...nslator/InstanceSegmentationTranslatorFactory.java	14.28% <0.00%> (-3.90%)	⬇️
.../modality/cv/translator/YoloTranslatorFactory.java	8.33% <0.00%> (-1.67%)	⬇️
...i/djl/modality/cv/translator/YoloV5Translator.java	5.69% <0.00%> (ø)
...odality/cv/translator/YoloV5TranslatorFactory.java	8.33% <0.00%> (-1.67%)	⬇️
...pi/src/main/java/ai/djl/ndarray/BytesSupplier.java	54.54% <0.00%> (-12.13%)	⬇️
...pi/src/main/java/ai/djl/repository/Repository.java	83.33% <ø> (ø)
...l/training/loss/SigmoidBinaryCrossEntropyLoss.java	64.00% <0.00%> (ø)
... and 209 more

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