Readme: improve consitency of compilation steps

Cross-Compilation steps and documentation building in Readme.md use "meson setup build" but general compilation steps currently use manual creation of build directory. This patch aims to make the general compilation steps consistent with the rest to increase understandability.

Currently compilation steps use two different types of methods, manual build directory creation and using meson setup build to directly create the build directory. This potential makes the new user who wants to build docs or cross compile confused about which step of compilation the user is in. This patch aims to make these steps clear.