doc: add default bitcoin.conf locations
doc: add default bitcoin.conf locations

Summary:
Added default bitcoin.conf data directories and paths
Added example bitcoin.conf link

Backport of Core [[https://github.com/bitcoin/bitcoin/pull/16621 | PR16621]]

Test Plan: Proofreading and testing link to example works

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7919

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