meson: fix building technical and howto docs

meson: fix building technical and howto docs

When our asciidoc files were renamed from "*.txt" to "*.adoc" in
1f010d6bdf7 (doc: use .adoc extension for AsciiDoc files, 2025-01-20)
the "meson.build" file in "Documentation" was updated but the
"meson.build" files in the "technical" and "howto" subdirectories were
not. This causes the meson build to fail when configured with
-Ddocs=html. Fix this by updating the relevant "meson.build" files.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Acked-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>