[WIP] Remove old dask-cudf documentation and add README

[rjzamora]

Closes #6583

TODO:

• Check that deleted content is covered elsewhere (otherwise the page should be replaced, rather than deleted).
• Clean up simple README and make sure proper links are included.

[GPUtester]

Please update the changelog in order to start CI tests.

View the gpuCI docs here.

[codecov]

Codecov Report

Merging #6665 into branch-0.17 will decrease coverage by 0.02%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           branch-0.17    #6665      +/-   ##
===============================================
- Coverage        82.33%   82.31%   -0.03%     
===============================================
  Files               94       94              
  Lines            15369    15366       -3     
===============================================
- Hits             12654    12648       -6     
- Misses            2715     2718       +3     

Impacted Files	Coverage Δ
python/cudf/cudf/utils/gpu_utils.py	53.65% <0.00%> (-4.88%)	⬇️
python/cudf/cudf/core/abc.py	87.23% <0.00%> (-4.26%)	⬇️
python/cudf/cudf/_fuzz_testing/fuzzer.py	0.00% <0.00%> (ø)
python/cudf/cudf/utils/hash_vocab_utils.py	100.00% <0.00%> (ø)

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[kkraus14]

@rjzamora should we close this PR?

[rjzamora]

@rjzamora should we close this PR?

Ah, sorry. This fell off my radar. It is probably fine to close this and replace it with something fresh for 0.19. I wouldn't consider this a huge priority.