Guides: graph analytics (views and algo.*) and time series, in SQL and openCypher #388

Following the DSL-first design, two engine areas are reachable from Python only through queries and have no guide: graph analytics (Graph Analytical Views: CREATE GRAPH ANALYTICAL VIEW, update modes, and the algo.* procedures such as PageRank, WCC, shortest paths, called from openCypher) and time series (CREATE TIMESERIES TYPE, inserts and the async append_samples path, ts.* functions such as ts.timeBucket, downsampling, retention). Add docs/guide/graph-analytics.md and docs/guide/timeseries.md (or extend existing pages if that fits the nav better) with short, runnable db.query/db.command examples, each covered by a test that runs it and asserts the answer, plus the mkdocs nav, the coverage doc and the AGENTS.md table row.