De-Coreify build documentation

Summary: Change Core-specific links and wording in the build documentation to be consistent with Bitcoin ABC. Still a few spurious core links, but they seem to be for scripts or other resources that may still be useful for Bitcoin ABC.

Test Plan: Read the docs

Reviewers: #bitcoin_abc, deadalnix, freetrader, awemany, O1 Bitcoin ABC

Reviewed By: #bitcoin_abc, deadalnix, O1 Bitcoin ABC

Differential Revision: https://reviews.bitcoinabc.org/D210