docs: split the README into a user page, an architecture page and a citations page

[rahlk]

Closes #416.

• README restructured around the user: Installation, Quick Start, Query Surface (one row per accessor family), Backends (selection by config type, the analyzer table), Read Next, Contributing, Citation, Maintainers. The hand-maintained table of contents and the per-language section are gone; nothing was invented.
• docs/architecture.md is the contributor page: factories in core.py, backend selection by the type of backend=, models and backends per language, shared result types. Linked from CONTRIBUTING under "Repository layout".
• docs/citations.md holds the Cited By list, moved verbatim.
• docs/agent-api-reference.md gains two notes agents keep hitting: project_path rules and the analysis cache under cache_dir (the Python analyzer's virtualenv lives there, so the first run is slower); Java call graphs need a JDK with javac from level 2 up, else the run exits 0 with declared edges only.

Checked before opening. Every relative link in the five touched files resolves; the mermaid block parses as a single fence. Each factual claim was read against the code: callers_of returns SliceNode with file / line / callable; get_comments is on the Java facade only; the Java backend logs an incomplete level at WARNING; project_path must be an existing directory on every backend; codeanalyzer-python's virtualenv is cache_dir/<project>/virtualenv; the cldk[all] extra exists; requires-python >= 3.11; the legacy entry emits DeprecationWarning; the analyzer table's Scalpel / RTA / def-use linker descriptions match the analyzers' own docs.

Out of scope, pre-existing. CONTRIBUTING links CODE_OF_CONDUCT.md, which exists neither in this repo nor at the org config repo's root. Left as is.

Docs only. Merge-only on the ship decision: the docs are not built from a tag.