Add pdf library

[StefanSchroeder]

This is a simple PDF library.

https://github.com/vlang/pdf

Could be put in Text or Graphics.
Or get its own section.

[vkvbit]

@StefanSchroeder I think it should be in Editor subsection after all its a pdf creation and editing module. I have created a pull request to it. Let's wait for maintainer to merge the PR.

[ArtemkaKun]

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