`docs/reference/*' was moved one directory up in commit 237fed6.
* builds/unix/unix-def.in (PIP): Remove variable.
* configure: Create `docs' directory and copy assets from
`docs/markdown'.
* docs/README: Output directory is `reference'.
Python versions < 3.5 have reached end-of-life and as such, no
security or bug fixes will be provided for those versions. See
https://devguide.python.org/#status-of-python-branches
for more information.
* Jamfile (RefDoc): Add `site' parameter.
* builds/detect.mk (std_setup): Update Python version requirement.
* builds/freetype.mk (refdoc-venv): Use pip as `python -m pip'.
* builds/unix/ax_compare_version.m4,
builds/unix/ax_prog_python_version.m4: Macros to detect Python
version. New files.
* builds/unix/configure.raw: Check for Python >= 3.5 and remove
check for `pip'.
* docs/CHANGES, docs/INSTALL.GNU, docs/README: Updated.
