DOC-9632 Serverless Essential Metrics

…s pages, particularly Request Units metrics, (b) promoted General Tips section to header 2 and created sections for types of resource usage, (c ) incorporated recommendations from internal wiki page.

…es, add link to changefeed(s).

…ge table statistics and row_level_ttl files, added links. (2) in metrics-essential-serverless, added anchors to metric names.

…les, added links and updated formatting.

…d files, added div with anchor link. (2) In metrics-usage tenant.consumption files, added links.

…ential-serverless.md, added See also section. (2) In metrics-request-units.md, added links. (3) In serverless-resource-usage.md, added links.