Remove installation instructions

As per buildstream#1364, it would be good for the installation instructions to live in the documentation, so that each branch/release can be associated with installation instructions that are correct for that branch / that release.

Once the install instructions are in place in the docs, we can delete them from the website.

This fits with a larger goal to streamline and improve the website.

UPDATE, following discussions on !143 (merged), we are keeping a reduced set of install instructions on the website, for package installation. (ie installing from repositories with a package manager).

NOTE: this is a subsection of #31 (closed), which !143 (merged) is intended to resolve.
If !143 (merged) is likely to merge soon, then this issue will be redundant.
Otherwise, we could consider addressing this issue individually, as soon as buildstream!1999 (merged) merges.

UPDATE: !143 (merged) has now merged and #31 (closed) is closed. This issue can be closed too.