Documentation: refactor "howto-index.sh" for out-of-tree builds

The "howto-index.sh" is used to generate an index of our how-to docs. It
receives as input the paths to these documents, which would typically be
relative to the "Documentation/" directory in Makefile-based builds. In
an out-of-tree build though it will get relative that may be rooted
somewhere else entirely.

The file paths do end up in the generated index, and the expectation is
that they should always start with "howto/". But for out-of-tree builds
we would populate it with the paths relative to the build directory,
which is wrong.

Fix the issue by using `$(basename "$file")` to generate the path. While
at it, move the script into "howto/" to align it with the location of
the comparable "api-index.sh" script.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>