Document how to create persistent archive with `bst source push`

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/1350)
In GitLab by [[Gitlab user @juergbi]](https://gitlab.com/juergbi) on Jul 6, 2020, 08:45

The following discussion from !1977 should be addressed:

- [[Gitlab user @sstriker]](https://gitlab.com/sstriker) started a [discussion](https://gitlab.com/BuildStream/buildstream/-/merge_requests/1978#note_372884174):

> I think this is excellent functionality. We should think about _documenting_ how to create a persistent archive from which you can always repeat a build without having to do source mirroring, but purely leveraging a persistent RA and CAS. This will result in being able to repeat builds even without the host tools needed by the Source plugins.

Start by reading the linked discussion from !1977 and the referenced merge-request discussion, then inspect the `bst source push` entry point and existing BuildStream documentation. Done means documenting how to create a persistent archive that supports repeatable builds using a persistent RA and CAS, without source mirroring or host tools required by source plugins.

Written by the indexing model from the issue text.