Refocus installation instructions on Bazelisk

I’ve noticed that [the installation guide](https://docs.bazel.build/versions/master/install-os-x.html) is focused on installing Bazel directly into a system. However, it seems like Bazelisk is a preferred way of managing Bazel versions, like the Gradle wrapper is. Shouldn’t we refocus the documentation on using Bazelisk instead, omitting direct Bazel installation guides? I think it might push users into the right direction. I can be completely wrong about this however since I’m very new to the scene. I can work on it if it makes sense.

Start with the linked macOS installation guide and review how it presents direct Bazel installation. Compare that guidance with the issue’s proposal to center Bazelisk, then update the relevant installation documentation so the preferred approach is clear and the completion criteria are consistent guidance across the affected instructions.

Written by the indexing model from the issue text.