[docs] (2/2) Fix generation of API documentation (#56745).

Creating the API Reference in the (new) `reference' sub-directory is
consistent with other documentation sub-topics, such as `design',
`glyphs' and `tutorial'.

This patch fixes broken hyperlinks in the documentation pointing to
and from the API Reference.  It also allows web assets to load from
their relative paths.

* builds/freetype.mk (DOC_DIR): Adjust.
(refdoc, refdoc-venv): Add `--site' argument.

* builds/toplevel.mk (do-dist): Updated.
3 files changed