doc: add default bitcoin.conf locations

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

Backport of Core 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