Multiple fixes

[tudorbarascu]

Fixes multiple problems (see #12 ) and warnings that the build outputted.
The structure wasn't cohesive anymore and now it's fixed.
I tried splitting into multiple commits but this is all that I could manage.

@sjib I tried to keep the structure you implemented but there were some duplicates that I had to remove.

The documentation builds now but there are 5 images ( see #12) that are linked in the docs but not pushed.

Also, there were some duplicated labels. E.g. the .. _Admin Guide: and _QGEP User Guide labels were pointing at multiple chapters which breaks the conventions as one link cannot go to multiple chapters.

[m-kuhn]

I guess this chapter was a duplicate?

[tudorbarascu]

They were almost duplicates, one of them didn't had the lines that came with the 0aac35a commit as seen by:

web@thinkpad:~/qgep_docs$ diff ./en/installation-guide/system/workstation.rst ./en/admin-guide/installation-guide/system/workstation.rst
26,32d25
< .. attention::
< 
<  On Windows, you need to save ``pg_service.conf`` in Unix format in order to work.
<  One way to do it is to open it with `Notepad++ <https://notepad-plus-plus.org/>`_
<  and ``Edit --> EOL Conversion --> UNIX Format --> File save`` .

I kept the newest version, of course.

[tudorbarascu]

I also added numbering to chapters and sub-chapters as I think it's an improvement.
Please say if you don't want them numbered now.

[m-kuhn]

It will be an improvement in general. And in case we find any regressions we can fix them up later.

Thanks a lot!