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

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

The "api-index.sh" script generates an index of API-related
documentation. The script does not handle out-of-tree builds and thus
cannot be used easily by Meson.

Refactor it to be independent of locations by both accepting a source
directory where the API docs live as well as a path to an output file.

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