[Docs] UX - Update "description" in the file metadata to improve user experience

The metadata in each documentation topic includes a description: which is displayed in search results (both Grafana Labs search and in Google search results). This description field in the metadata has minimal information in much of the Loki docs set. It would be helpful to users to take a pass through the documentation and attempt to write some meaningful text for the descriptions.

https://grafana.com/docs/writers-toolkit/writing-guide/front-matter/

Additional context The bold heading is the menuTitle metadata (or from title if menuTitle has not been defined) and the text below is from the description metadata.