Fix missing files in CTAN zip created by `l3build ctan`

[muzimuzhi]

Motivation for this change

This PR adds back ./source directory and missing files including licenses and more under ./doc to CTAN zip.

Therefor it makes the content and structure of CTAN zip almost the same as that in pgf_3.1.9.tds.zip, generated by generate_CTANzip() defined in build.lua, with a flattened doc directory structure.

I will show the specific changes perhaps in the form of git diff on tree <dirname> outputs to show what's really changed.

Fixes #1240

Checklist

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• [x] Code changes are licensed under GPLv2 + LPPLv1.3c
• [x] Documentation changes are licensed under FDLv1.2

[muzimuzhi]

... in the highly unlikely case that anyone actually want to use these files, they can also download them from GitHub.

I never used such utility.

If the decision is to drop source directory, then we might want to add some words in sec. 37 "Writing Graph Drawing Algorithms in C" to point users to ./source/generic/pgf/c directory in GitHub repo.

Interesting all Makefiles in source were not packed into zip by generate_CTANzip(), so I guess the source directory shipped to TeX Live had been incomplete for years. https://github.com/pgf-tikz/pgf/blob/b561e46fbc36215fe8b8459bc2234d1b0670f654/build.lua#L323-L326

[muzimuzhi]

Interesting all Makefiles in source were not packed into zip by generate_CTANzip(), so I guess the source directory shipped to TeX Live had been incomplete for years.

I was wrong. The Makefiles under source were zipped.

[muzimuzhi]

The current status of this PR includes only ./doc files in CTAN zip, no ./source files.

I'd like to check whether the Makefiles in ./source/generic/pgf/c work as expected when installed in some <tds tree>/source, but didn't find the time/interest.