List cmake as a package to install in build instructions

[steviez]

Problem

#24147 bumped prost dependency version; the new version requires cmake. Without it, I got the following error building from source

  running: "cmake" "/home/.../.cargo/registry/src/github.com-1ecc6299db9ec823/prost-build-0.10.0/third-party/protobuf/cmake" 
"-DCMAKE_INSTALL_PREFIX=/home/steven_solana_com/solana/target/release/build/prost-build-6551c412a1f344fc/out" 
"-DCMAKE_C_FLAGS= -ffunction-sections -fdata-sections -fPIC -m64" "-DCMAKE_C_COMPILER=/usr/bin/cc" 
"-DCMAKE_CXX_FLAGS= -ffunction-sections -fdata-sections -fPIC -m64" "-DCMAKE_CXX_COMPILER=/usr/bin/c++" 
"-DCMAKE_ASM_FLAGS= -ffunction-sections -fdata-sections -fPIC -m64" "-DCMAKE_ASM_COMPILER=/usr/bin/cc"
"-DCMAKE_BUILD_TYPE=Debug"

  --- stderr
  thread 'main' panicked at '
  failed to execute command: No such file or directory (os error 2)
  is `cmake` not installed?

[steviez]

I was clicking without thinking on backports - we definitely don't need them as this is docs only

[CriesofCarrots]

Thanks for doing this!
Also, fyi I discovered that older cmakes (including the default version for ubuntu 18, cmake v3.10) hits a different error: https://buildkite.com/solana-labs/solana/builds/69769#abc1b1e6-14f9-40b2-858e-592175d33b04
I'll keep an ear out and we can document a msv if it seems to be a issue for others.