Document GLQL merge request and AI usage event analytics

What does this MR do and why?

Adds user docs for the two new GLQL analytics sources shipping in 19.3 (MergeRequest and AiUsageEvent), now that the compiler (@gitlab/query-language-rust 0.33.0), backend (!248034 (merged)), and presenters (!248035 (merged)) are in place.

• New page for merge request analytics, following the pipeline analytics page structure
• New page for AI usage events (an analytics-only source, like code suggestions)
• Data sources index updated: merge requests now shows analytics mode support, plus a row for the new source
• Field lists, operators, and defaults were verified against the compiler analyzers rather than the tracking issues, which had stale examples

A few notes:

• Neither source is feature flagged. The flags named in the issues were never created
• The GraphQL reference docs were already regenerated by the backend MR, so there's nothing to do there
• Navigation entries for the two new pages are drafted in docs-gitlab-com!2503, to merge after this MR

Author note (non-blocking): the duration rendering for timeToMergeQuantile and the user avatar rendering described in these docs come from !248035 (merged), so this shouldn't merge before that does.

Closes #592272 (closed) and #592280 (closed)