doc: move extensions.worktreeConfig to the right place

doc: move extensions.worktreeConfig to the right place

All config extensions are described in technical/repository-version.txt.
I made a mistake of adding it in config.txt instead. This patch moves
it back to where it belongs.

Since repository-version.txt is not part of officially generated
documents (it's not even part of DOC_HTML target), it's only visible
to developers who read plain .txt files. Let's include it in
gitrepository-layout.5 for more visibility. Some minor asciidoc fixes
are required in repository-version.txt to make this happen.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>