docs(slicing): Add mega-cluster and basic consumer setup

[ayirr7]

This PR is the first step towards furthering the docs for slicing (https://getsentry.atlassian.net/browse/SNS-1893)

Next steps are:

• As sliced scheduler and executor work are completed, fill in the section about subscriptions
• Potentially add a diagram for the mega cluster
• Explain the end to end for slicing, slicing boundaries

[codecov-commenter]

Codecov Report

Base: 91.71% // Head: 92.22% // Increases project coverage by +0.51% 🎉

Coverage data is based on head (15a7954) compared to base (6917371).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3433      +/-   ##
==========================================
+ Coverage   91.71%   92.22%   +0.51%     
==========================================
  Files         724      724              
  Lines       33782    33782              
==========================================
+ Hits        30983    31156     +173     
+ Misses       2799     2626     -173     

Impacted Files	Coverage Δ
snuba/datasets/storages/factory.py	95.95% <0.00%> (+2.02%)	⬆️
snuba/datasets/slicing.py	88.88% <0.00%> (+11.11%)	⬆️
snuba/cli/__init__.py	86.36% <0.00%> (+18.18%)	⬆️
snuba/utils/streams/metrics_adapter.py	83.33% <0.00%> (+83.33%)	⬆️
snuba/consumers/consumer_builder.py	92.59% <0.00%> (+92.59%)	⬆️
snuba/cli/consumer.py	94.44% <0.00%> (+94.44%)	⬆️

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[onewland]

Thanks for doing this clean-up!