doc: Add detailed info about Bitcoin Core files
doc: Add detailed info about Bitcoin Core files

Summary:
This is a backport of Core [[https://github.com/bitcoin/bitcoin/pull/16983 | PR16983]]

fee_estimates.dat was remove because it is not used.

Test Plan: Read.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D8009

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