doc: Cleanup up observability docs

Under the Gitaly docs folder, there are two files dashboards.md and observability.md. observability.md seem to be a subset of what is in dashboards.md apart from a small difference**.** Also naming of the two files are bit confusing because content of observability.md is just about dashboards while content of dashboards.md is about general observability such as logs, metrics, dashboards.

I suggest removing the observability.md after moving the small difference to the dashboards.md and renaming the dashboards.md to observability.md**.** Or in another words, move the contents of dashboards.md to observability.md, and remove the dashboards.md.