extras: update sphinx-to-github

Signed-off-by: David Aguilar <davvid@gmail.com>
docs: support symlinks in conf.py

Signed-off-by: David Aguilar <davvid@gmail.com>
docs: use `install` instead of `cp` and rework targets

Factor out an install-files target that does not require
sphinx to be installed, which we can later use when we
update the Homebrew formula.

Use `install` to install files instead of `cp`.

Signed-off-by: David Aguilar <davvid@gmail.com>
docs: add phony target declarations

Signed-off-by: David Aguilar <davvid@gmail.com>