[DOC] document import-module and friends

We don't have any documentation for ndk-build's `import-module` feature AFAIK. It comes up in some of the tests/samples for adding cpufeatures and the like, but it's not described anywhere.

Start by searching the ndk-build documentation and the tests/samples that use `import-module`, including the cpufeatures examples. Document how `import-module` and related commands work, using those existing examples to establish the expected scope and usage. Done means the feature is described clearly with relevant examples.

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