Docs: add a usage guide with use cases and step-by-step guides for running a simulation

[amvlab]

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

The current documentation is deployment-focused. The README covers installation and operations thoroughly (Docker Compose, prebuilt release, build-from-source, environment variables, BlueSky ports, offline PMTiles basemap, and the integrated edition), and script/README.md documents the navdata build pipeline. But there is no end-user guide for actually running a simulation once WebATM is up.

A new user can get the web UI running at http://localhost:8082, but from there nothing explains how to connect to BlueSky, load/run a scenario, create and control aircraft, issue commands, or use the map tools. The feature list in the README names capabilities (command palette, console map picker, aircraft creation, routes/shapes, multi-node, 3D, follow mode) without showing how to use them. The result is a steep "now what?" cliff right after install.

Describe the solution you'd like

Add user-facing usage documentation — a Usage / User Guide with concrete use cases and step-by-step guides to run a simulation. This could be a new top-level section in the README and/or a docs/ directory linked from it (and screenshots/GIFs where they help).

Proposed step-by-step guides:

1. Your first simulation in 5 minutes — from a fresh start to a moving aircraft on the map (connect → load a demo scenario or create one aircraft → run → observe).
2. Connecting to a BlueSky server — standalone: Settings → server IP → Connect (note ports 11000/11001 and BLUESKY_SERVER_HOST); integrated: the in-app Start button.
3. Running a scenario (.scn) file — upload/organize files via the in-app file manager and run them (incl. the IC command flow).
4. Creating & controlling aircraft from the UI — the Create Aircraft modal + click-to-place on the map; single-click to fly-to, double-click to follow; reading the aircraft info panel; issuing commands (e.g. HDG/ALT/SPD) from the console with tab-completion and the command palette.
5. Using the map tools — drawing routes and shapes, and picking coordinates with the console map picker.
6. Multi-node simulations — spawning and switching between parallel nodes.
7. Customizing the display — labels, icons, trails, colors/styles, 2D ↔ 3D models, and Web Mercator ↔ globe projection.
8. Integrated edition: managing the bundled server — Start / Stop / Restart / Kill and the live server-log tab.

Proposed use cases (the "why" framing each guide):

• A quick classroom/demo of an ATM scenario for teaching or evaluation.
• Validating a hand-authored or generated .scn scenario file end-to-end.
• Running an offline/air-gapped demo using the local PMTiles basemap + navdata overlay.
• Driving a single-container deployment via the integrated edition's in-app server controls.

Describe alternatives you've considered

• Keeping everything in the README vs. splitting usage into a docs/ folder linked from the README (likely cleaner as the guides grow).
• Text-only vs. adding short screenshots/GIFs (a few visuals would help a lot for the map-interaction guides).
• Relying on the live demo and BlueSky's upstream command reference instead of writing our own — but neither walks a user through the WebATM UI specifically.

Additional context

• Existing docs for reference: README.md (install/deploy/config) and script/README.md (navdata pipeline).
• BlueSky stack commands referenced in the guides can link to the upstream BlueSky / amvlab/bluesky docs rather than being re-documented here.
• Guides should call out the standalone vs. integrated differences where the workflow diverges (e.g. connecting to a server vs. clicking Start).