Reintroduce installation instructions to buildstream docs

Summary

Installation instructions from https://buildstream.build are fragmented and confusing, partly because they're an attempt to use the same set of instructions for both buildstream1 and buildstream2.

Installation instructions should be part of the documentation, maintained from inside the buildstream repository. That way, they can be kept up to date and accurate for the specific branch or tag that they are associated with.

Buildstream 1 already has installation instructions in the docs (https://docs.buildstream.build/1.4.3/main_install.html). So we only need to do this for master.

The installation instructions should appear as part of https://docs.buildstream.build/master.