Update logging docs for VictoriaLogs migration and app logs API corrections

[lesliebc]

Summary of changes

Updates logging docs to reflect migration from Quickwit to VictoriaLogs. References to Quickwit are replaced with VictoriaLogs, and our log retention period is updated to 7 days.

Also makes a few changes to the app logs endpoint doc:

• Changes API response format from newline-delimited JSON stream to JSON:API document
• Replaces start_time with next_token in supported query params
• Documents endpoint's default behaviour

[kcmartin]

Left a few comments and a suggestion

[kcmartin]

next_token is described as both "a starting timestamp" (user-suppliable) and "pass it in your next request to page forward" (opaque cursor). Can you confirm which is the intended use, and update as needed?

• If users can construct one themselves to backfill from an arbitrary moment: add an example value to the bullet (the old doc had start_time=2023-08-01T00:00:00Z what's the nanosecond equivalent look like?).
• If it's only an opaque cursor returned by the previous response: drop "or set a starting timestamp" from the intro sentence on line 40, and rewrite the bullet as something like "an opaque paging cursor returned in meta.next_token of the previous response."

Either way, the rename from start_time (ISO-8601) to next_token (nanosecond Unix) is a breaking change for anyone using the old param. A one-line note calling that out would save existing users some debugging.

[lesliebc]

next_token can be used both ways that you described. I think it's primarily a cursor, but you can also approximate backfilling by passing in your own timestamp as next_token. Could the usage be clearer? I also added an example for that param like you suggested. Thanks!

About the previously documented start_time, I don't know that this was working before. I don't see that param in the bubblegum or web code, but I could've missed it. next_token has been in the logging code for a long time - I think this was always the supported way to backfill and page through logs, before and since migrating to VictoriaLogs.