docs: fix building with meson 0.61.1

I got the following errors when trying to build with meson 0.61.1:
[1] docs/polkit/meson.build:18:6: ERROR: gnome.gtkdoc keyword argument 'gobject_typesfile' was of type array[File] but should have been array[str]
[2] docs/polkit/meson.build:18:6: ERROR: gnome.gtkdoc keyword argument 'html_assets' was of type array[File] but should have been array[str]

Writing polkit-1.types explicitly because there is only one such file.
Moved html_images into another file because this variable is used only there.

I had to prefix image paths with "../" because otherwise I got:
[3] No such file or directory: '/tmp/abf/rpmbuild/BUILD/polkit-0.119/docs/polkit/polkit-architecture.png'

It is a quick fix that I had to make just to build polkit with a fix of CVE-2021-4034 applied,
please fix meson in a better way if needed.

Signed-off-by: Mikhail Novosyolov <m.novosyolov@rosalinux.ru>
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