Move Dokka setup to a separate plugin and make samples discovery flexible

[fzhinkin]

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Currently, Dokka configured separately in both core and bytestring modules. And in each of these configs paths to files containing samples are listed explicitly.

It would be nice to:

• use file pattern instead of paths to particular files (it should be something like **/test/samples/**/*.kt)
• move common configuration logic to a separate plugin in build-logic