feat(libxml2): move more docs to docs package

[shyim]

Make the image smaller by removing docs

Fixes: shyim/wolfi-php#519

Related:

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[ajayk]

@shyim needs linting fixes

[shyim]

@ajayk fixed

[shyim]

@ajayk I can switch the clone to https://github.com/GNOME/libxml2 . wdym?