Docs: Add guidance about time formatting in dashboards

From [Slack conversation](https://raintank-corp.slack.com/archives/C5PG2JK8W/p1732265979629209) with @mjseaman: > I was just trying to format a time field in a table to convert a timestamp into a “fromNow” format, e.g. “11 hours ago.” I did lots of hunting between transformations, query languages, and panel settings and traversed our docs, chatgpt, and community forums before discovering that I could use Units in a cell override to do what I wanted . This strikes me as a good idea for a Guide or a little page about “working with time :clock1:.” This is not a time-sensitive request :trollface: but I bet it would help lots of users. Where would be a good place for me to write this down? It might be something we could add as an example here: https://grafana.com/docs/grafana/latest/panels-visualizations/configure-overrides/ This might also be a good blog post per @timlevett