#319/carla 0.9.16 windows build guide and doc build cleanup

[yunlishao-vibe]

Summary

The CARLA 0.9.16 Windows build guide

doc/Carla_0916_Windows_building.md, written from a build actually carried out at tag
0.9.16 (C:\src_ext\CarlaSrc_0_9_16), not from the upstream docs.

The reason it needs its own guide rather than a note on the 0.9.15 one: 0.9.16
hardcodes the VS2022 toolset (Setup.bat --boost-toolset msvc-14.3, where 0.9.15 passed
it through a variable), so the compiler changes and most of the 0.9.15 workarounds stop
applying. The guide covers:

• the five patches the tree needs to build at all — a trailing-backslash quote escape that
  silently breaks cd in 7 installer scripts, a bootstrap.bat vc141 call against a
  msvc-14.3 default, python -m build's isolated venv, and an MSVC ICE plus a COFF
  section-limit overflow on boost 1.84;
• two Windows shell traps — driving the build from Git Bash breaks vcvars64.bat, and
  launching from PowerShell breaks bootstrap.bat via an inherited environment variable;
• that make setup and make PythonAPI exit 0 while having failed, so artifacts must
  be checked rather than return codes.

It also documents that make package does not work — UE 4.26's AutomationTool crashes
(0xC0000005) under the VS2022 toolchain 0.9.16 requires — and that the way around it is
to launch through the engine's UE4Editor.exe, the way CARLA itself does
(BuildCarlaUE4.bat:205), which loads uncooked content. That path is verified: 25 NPCs
spawned with 24 driving under Traffic Manager, four synchronous client sessions, no errors
in the server log.

FIXS still targets 0.9.15. Nothing here changes what FIXS builds against.

Doc cleanup

All of this came out of actually building the docs, which had not been done in a while.

• Removed doc/Carla_0915_Windows_Fixes.md. It is a 1.5 KB PDF with a .md
  extension whose entire content is the string "Placeholder for Carla 0915 Windows
  Fixes". Sphinx reads it as source and emits undecodable-character warnings on it.
• Filled in doc/index.rst. Eight documents existed in the tree but appeared in no
  toctree, so they were unreachable from the navigation and Sphinx warned on every one.
  Orphan count 8 → 0. Adds a Building from Source section and completes XIL Bench
  and Development.
• Renamed FIXS_message flow.md → FIXS_message_flow.md — a space in a Sphinx
  docname is fragile. Nothing referenced the old name.
• Fixed seven URL-encoded image paths in CarMakerDoc.md (img%2F → img/) that left
  those images broken in the rendered docs.
• Cross-linked the 0.9.15 guide, the 0.9.16 guide and CARLAdoc.md so a reader lands
  on the right one, and corrected Util/InstallerWin → Util/InstallersWin.

Sphinx warnings 51 → 31, orphans 8 → 0.

One thing worth knowing

Building the docs on Windows segfaults, part-way through writing output... and
sometimes surfacing first as a nonsense TypeError from inside Sphinx or markdown-it. It
is stack exhaustion, not a bad page: docutils recurses deeply per document and Windows
gives the main thread far less stack than Linux, so past a certain number of written
documents the build no longer fits. Adding documents to the toctree is what tipped it over.

Verified by building on a thread with a 64 MB stack, which succeeds cleanly. ReadTheDocs
builds on ubuntu-22.04 (.readthedocs.yml) and is not affected. The workaround is
written up in ContributionGuide.md so the next person does not have to rediscover it.

Residual uncertainty: I could not run a Linux Sphinx build here, so "RTD is unaffected"
rests on the platform stack-size difference plus the fact that the base branch already
builds on Windows at 21 written documents. The RTD build on this PR settles it directly.

Related Issues

Relates to #319 (CARLA 0.9.16 investigation and migration — Backlog). Does not close it;
#319 covers the FIXS migration itself, which this PR deliberately does not touch.

Environment

• Python version: 3.10.11 (CARLA build), 3.10.11 + Sphinx 7.4.7 (docs)
• CARLA: 0.9.16 (tag), Unreal Engine 4.26 CARLA fork, Visual Studio 2022 / MSVC 14.43

Checklist

• Code compiles/runs as expected — docs build clean; the 0.9.16 build and server launch are the subject of the guide and were both carried out
• Tests pass locally — Sphinx build succeeds, 0 orphans
• Documentation is updated (if applicable)
• Issue linked above