doc: Add anchors.dat to files.md
Summary:
This is a backport of core#17428 [7/7]
https://github.com/bitcoin/bitcoin/pull/17428/commits/a490d074b3491427afbd677f5fa635b910f8bb34
Depends on D10512
Test Plan: proofreading
Reviewers: #bitcoin_abc, majcosta
Reviewed By: #bitcoin_abc, majcosta
Differential Revision: https://reviews.bitcoinabc.org/D10513