Until now, these types of files were being reported by the 'dh_missing'
utility.

With this commit, the PNG (and place-holder TXT) images are being installed
in 'info/gnuastro.info' in case a reader of the Info manuals would like to
see the images. But the EPS and PDF versions of the images are added to
'debian/not-installed' since they aren't necessary in this
scenario. Similarly, Gnuastro's logo (an SVG file), and the README files
are also added to 'debian/not-installed'.

As a result of this change, there are no warnings from 'dh_missing'.

This was all done with the guidance and help of Ole Streicher.

Until now the instructions to install the man and info pages were not
correct (for example saying 'doc/*.info*' instead of
'usr/share/info/*.info*').

With this commit, they have been corrected (as suggested by 'dh_missing')
and they no longer show up after a 'pdebuild' (they are treated
successfully).

Following, this, I also corrected some other files that were being reported
by 'dh_missing' (like 'gnuastro.la'). But for some reason the files in
'debian/not-installed' are not being seen, so we see some warnings about
those.

Until now, the LGPL license of these files (that are imported from Gnulib)
was missed in the 'debian/copyright' file.

With this commit, it has been added, along with a comment on the purpose of
these files.

This issue was reported by Thosten Alteholz.