Put the "related packages" into a table, and clarify that Octavian is a CPU library

[DilumAluthge]

This pull request does a few different things:

1. It puts the list of "related packages" into a table
2. In the table, it adds columns that show which packages work on the CPU and which work on the GPU
3. Clarify in the README and the documentation that Octavian is a CPU library

[codecov]

Codecov Report

Merging #69 (46070c4) into master (369aafe) will decrease coverage by 0.16%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #69      +/-   ##
==========================================
- Coverage   83.83%   83.66%   -0.17%     
==========================================
  Files          12       12              
  Lines         600      600              
==========================================
- Hits          503      502       -1     
- Misses         97       98       +1     

Impacted Files	Coverage Δ
src/block_sizes.jl	94.82% <0.00%> (-1.73%)	⬇️

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