Add possibility to include created plots in PDF documentation

[numeredev]

DESCRIPTION

Is your change request related to a problem? Please describe.
Although it is in principle possible to include images in the PDF documentation using LaTeX commands, that is quite tedious as it requires to give the absolute path and does not allow using placeholders.

Describe the solution you'd like
Provide at least a simple doc keyword like \insertplot "FILE" "CAPTION", which will include the graphic FILE at its position using the caption CAPTION. This keyword shall not be part of the internal documentation or converted to something like [Image].

Describe possible alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered, if any.

Additional context
Add any other context or screenshots about the feature request here.

(Do not write below this line)

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DEVS' SECTION

ANALYSIS

(Describe, what's the issue and which changes have to be made)

IMPLEMENTATION STEPS

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• Implement the necessary changes in a new branch created here on GitHub
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DOCUMENTATION STEPS

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• Update the changes log
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• Create or update the documentation articles (*.NHLP and *.NDB files, if needed)
• Update the language strings (*.NLNG files, if needed)

PULL REQUEST

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