android: [docs][NewPM] Add docs for writing NPM passes

android: [docs][NewPM] Add docs for writing NPM passes

Porting of commit
c2590de30df2 ("[docs][NewPM] Add docs for writing NPM passes")

Fixes the following building error:

ld.lld: error: undefined symbol: llvm::HelloWorldPass::run(llvm::Function&, llvm::AnalysisManager<llvm::Function>&)
>>> referenced by PassManagerInternal.h:85 (external/llvm-project/llvm/include/llvm/IR/PassManagerInternal.h:85)
>>>               PassBuilder.o:(llvm::detail::PassModel<llvm::Function, llvm::HelloWorldPass, llvm::PreservedAnalyses, llvm::AnalysisManager<llvm::Function> >::run(llvm::Function&, llvm::AnalysisManager<llvm::Function>&)) in archive out/soong/.intermediates/external/llvm-project/llvm/lib/Passes/libLLVM12Passes/android_x86_64_core_static/libLLVM12Passes.a