Slim down the website - remove and reorganize content

The buildstream website should be reduced in size and redesigned.

There should ideally be only one page of content, answering the most fundametal questions "What is buildstream?", "Why should I use it?".

There should be with links to external locations (like the full docs), to answer other questions (how do I install? How do I contribute? How do I use it?)

For instance we can remove:

The 'About' page - information 'about' buildstream, should be what's on the main (/only) page. The 'News' section - This has only been updated twice. Having only two entries makes it look like the project has stalled. It would be better not to have a news section at all for now. Installation instructions - These can be moved to the docs. See buildstream#1364 & #30 (closed)

UPDATE: following discussions on !143 (merged) we are now keeping the NEWS pages, and a reduced install page. The install page only has instructions for package install, with external links for container-install, and install-from-source.