docs: split the README into a user page, an architecture page and a citations page; two agent-guide notes

[rahlk]

Is your feature request related to a problem? Please describe.

README.md on release/2.0 is doing three jobs at once. It is the user's first page, but it also carries a contributor-facing Architecture section (the module map, which only matters to someone changing the SDK) and a fourteen-entry Cited By list, under a hand-maintained table of contents. The user-facing part — what the query surface is, which backend to pick, where to read next — is the part that gets buried.

Describe the solution you'd like

• README restructured around the user: Installation, Quick Start, Query Surface, Backends, Contributing, Citation, Read Next, Maintainers. Table of contents and the per-language section dropped; the content moves rather than disappears.
• docs/architecture.md — the contributor page: CLDK factories in core.py, backend selection by the type of backend=, per-language models and backends, the analyzer each one drives. Linked from CONTRIBUTING under a new "Repository layout" heading.
• docs/citations.md — the Cited By list, with the CLDK paper as the anchor.
• docs/agent-api-reference.md gains two operational notes that agents keep hitting: project_path is optional with Neo4jConnectionConfig and must exist and be a directory on every backend; local runs write under cache_dir (default <project>/.codeanalyzer, one subdirectory per language, plus the Python analyzer's virtualenv), so the first Python run is slower and the directory belongs in .gitignore. And: Java call graphs need a JDK from level 2 up — without JAVA_HOME pointing at one with javac, the run exits 0 but degrades to declared call edges.

Describe alternatives you've considered

Not stated in the original issue.

Additional context

Scope boundary

• Docs only. No code, no test, no version.
• Every link in the four touched documents resolves to a file in the repo or to a public URL. No content is invented; the Cited By entries move verbatim.
• Merge-only on the ship decision: the docs are not built from a tag, so no release follows.

Definition of done

README, CONTRIBUTING and the agent guide render with every relative link resolving; docs/architecture.md and docs/citations.md exist and are linked from the README's Read Next section; the mermaid block in docs/architecture.md parses.

[rahlk]

Merged in #417 (9440b95) on release/2.0. README restructured around the user; the module map now lives in docs/architecture.md and the Cited By list in docs/citations.md; the agent guide carries the cache and JDK notes. Docs only, no release.