Improve GeoPDF export/import documentation

[havatv]

Description

With the merging of pull request #4437, the user manual was updated with initial documentation
of PDF / GeoPDF export/import.
The documentation is currently scattered around (layout export, map export).
A new section on GeoPDF support in QGIS should be considered.
It would be nice if someone with experience in GeoPDF export and import could improve
the GeoPDF documentation! Links to issues related to GeoPDF / PDF are included in #4437.

URLs for pages affected by the #4437 contributions:
https://github.com/qgis/QGIS-Documentation/tree/master/source/docs/user_manual/introduction/project_files.rst
https://github.com/qgis/QGIS-Documentation/tree/master/source/docs/user_manual/print_composer/create_output.rst

[ioalexei]

Currently there's a bit of documentation of GeoPDF under Map view and some under Print composer.

I'm happy to have a go at expanding the GeoPDF documentation; I think it would work best as a separate subsection in the Print composer > Export settings section.

So it would look like:

• 21.3.1 Export settings
• 21.3.2 Export as image
• 21.3.3 Export as SVG
• 21.3.4 Export as PDF
• 21.3.5 Export as GeoPDF

What do people think?

Links for context:

• North Road: QGIS loves GeoPDF
• GDAL: GeoPDF
• Adobe: Geospatial PDFs
• USGS: What is Geospatial PDF?
• GeoPDF blog archive
• GeoPDF vs PDF comparison table

[ioalexei]

Adding some notes before I start on this. Suggest:

• Adding GeoPDF info to Print Composer/Create Output page
• Minimising the GeoPDF content in the Map Views/Exporting the map view page and linking it to Create Output for more info
• When live, replace links to external content on GeoPDF with the CreateOutput page (which can have some links to external content if necessary)

[DelazJ]

@ioalexei sorry for not responding earlier. Plan looks good to me. Thanks

[ioalexei]

Coming back to this.

Note based on recenty changes to the docs - align with recent changes to refer to geospatial PDF rather than trademarked GeoPDF

060e5ca