docs: Add hash-table-caching documentation (facebookincubator#16554)

Summary:
Pull Request resolved: facebookincubator#16554

Add the hash-table-caching.rst doc

Reviewed By: xiaoxmeng

Differential Revision: D94303195

fbshipit-source-id: 4926d3f85311c82ee35b6f1741d6908c038e65bc

Author: shrinidhijoshi