docs: 📝 Add section 1.2 about running a simulation and the tests - #224

Part of #199 (section 1.2).

What this adds

A new page doc/getting_started/running_simulations.md, linked from the chapter page, covering the three points the issue lists for this section:

• Running a simulation: one script under src/simulations/, run from the repository root (example: the EKF localization simulation), with what the window and the terminal output show.
• --save-gif: which simulations write a GIF (about half of them define a GIF output), where the file goes (the simulation's own folder), and that these GIF files are the ones the gallery shows. Example with the Hybrid A* planner.
• show_plot and the tests: each script keeps its animation behind a show_plot flag, the tests call each main() with the flag off, and . run_test_suites.sh (pytest over test/) confirms the environment is complete.

It follows the writing guide: a search-oriented first sentence, one page per section, no theory layer (the page links to Chapter 2 for the simulator itself). The chapter page's note now says the chapter is being written, and section 1.2 links to the new page, so it is nested under the chapter in the sidebar.

Verification

$ sphinx-build -W --keep-going -b html doc doc/_build/html
build succeeded.   # zero warnings; sidebar nests 1.2 under Chapter 1

$ . run_test_suites.sh
110 passed in 305.77s

$ python src/simulations/localization/extended_kalman_filter_localization/extended_kalman_filter_localization.py
Simulation start!!
Simulation finished!!   # exit 0, animation window

$ python src/simulations/path_planning/astar_hybrid_path_planning/astar_hybrid_path_planning.py --save-gif
# writes astar_hybrid_search.gif and astar_hybrid_navigate.gif into the simulation's folder

Section 1.2 was claimed in #199.