Small improvements to Wikibase technical docs

[wetneb]

This slightly improves the overview of the architecture of the Wikibase extension.

@Abbe98 I could write a ton more (probably in subpages in the same section), is there any area that you would be interested in? I am thinking specifically about what you would need to tackle things like https://github.com/OpenRefine/OpenRefine/issues/1713

[wetneb]

Preview of the edited page: https://deploy-preview-5110--openrefine-docs.netlify.app/next/technical-reference/wikibase/architecture

[Abbe98]

@wetneb not for my own needs, but documentation that I could have found helpful:

1. Schema abstractions compared to Wikidata's conceptual model(+overview).
2. High-level overview of merging and merging strategies.

[wetneb]

Well noted, doing those in follow-up PRs.