Reintroduce install instructions to docs

ntroduces install instructions in to the docs, as per #1364.

Installation instructions were copied from https://buildstream.build/install.html

Instructions have been edited to be more directly relevant to Buildstream 2. The sections on package installation and tarball installation have been removed, since they can't currently be used for Buildstream 2. Other sections have had minor edits.

Changes made:

• Buildbox-instructions:
  • Fix docs in buildbox repo, point instructions to it
• PyPi section:
  • Removed references to "stable releases". At this time, all BuildStream 2 tags are dev pre-release, not stable releases.
  • Added a link to https://pypi.org/project/BuildStream/#history, instead of to the BuildStream releases page.
  • Added "--pre" option to the 'update from PyPi' subsection.
• Docker instructions:
  • Fix instructions in buildstream-docker-images repo, point documentation to it