Replace the remaining Knowledge Base links with their MariaDB
Documentation equivalents, including the 838 URLs in the help tables.
Only URLs change in fill_help_tables.sql.
Merging upward: fill_help_tables.sql conflicts at 10.11->11.4 and
11.8->12.3. At those two merges keep the target branch's version,
since 11.4, 11.8 and 12.3 each carry their own URL fix. From 12.3 to
13.1 and 13.1 to main it merges cleanly; take the incoming change.
.github/pull_request_template.md is deleted in 12.3; keep the deletion.
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