docs: Add project anatomy page and deepen the Server fundamentals section

[developerjamiu]

Deep review and rework of the Server fundamentals section, with every technical claim verified against the Serverpod 4.0.0-beta.0 source and a project created with the 4.0.0-beta.0 CLI.

New page

• Your Serverpod project: the section opener explaining what serverpod create generates. Starts with the three-package mental model and a short tree, with the full annotated workspace tree in a collapsible. Covers package relations (pub workspace, dependency direction), where user code lives vs generated code, the server entry point (bin/main.dart, run(), pod.start()), the three servers and their ports, the config and migrations directories, the .serverpod embedded-database directory, and the editor/agent files.

Reworked pages

• Running your server: the dev loop now includes the database leg (embedded PostgreSQL). Adds degraded-start recovery, the app launch panel (Ctrl+R), --docker, single-instance behavior, run-mode gating of app launches, and new "Run the server directly" and "Choose a server role" sections (monolith / serverless / maintenance semantics). Documents the key notation precisely: shortcuts are unshifted presses, Shift variants are written out, and Shift+M / Shift+P force a migration even past data-loss warnings. Notes that P is not shown in the bottom bar.
• Configuration: fixes the source-order contradiction in the overview and the websocketPingInterval example nesting (it is a top-level key; the nested form was silently ignored). Adds a Run modes section, embedded PostgreSQL under Database backends, the SQLite session-log limitation, dedicated secret environment variables with their precedence, and aligns the example with the scaffolded config/development.yaml. Secrets are restructured example-first.
• Modules: corrects the module reference example (AuthUser lives in serverpod_auth_core; references resolve by effective nickname only, and an override replaces the default), pins beta versions exactly, and rewrites the overview reader-first.

Reference and consistency fixes

• Configuration reference: corrected consoleEnabled/consoleLogFormat defaults, new Dart-only options (healthCheckInterval, experimentalDiagnosticHandlerTimeout), and password environment variables.
• Working with endpoints: generation guidance now leads with serverpod start's automatic generation; serverpod generate remains as the no-session path.
• Logging: corrected the console-logging default and retargeted a stale pointer; same pointer fix in Streaming and Scheduling configuration.
• How it works: links to the new anatomy page.