Refresh stale end-user and build documentation

[thedavidweng]

Brings the user-facing docs in line with the current state of the project. No code changes.

End-user downloads (README.md, docs/index.rst):

• Point Windows/macOS/Linux standalone downloads at the official frontend-sdl-cpp 2.0.0-pre1 release (covers all three platforms) instead of the outdated personal-fork previews.
• Qt frontend entry: Qt5-only/PulseAudio+JACK/broken -> Qt5+Qt6 with PipeWire/PulseAudio/JACK, requires libprojectM 4.x.
• Canonical app link updated to frontend-sdl-cpp; synced index.rst with the README (it had drifted: old release links, old preset pack, removed Todo section).
• Fixed broken logo URL in index.rst (missing docs/ path segment), unified the Discord invite, fixed the BUILDING.md link.

Build docs (BUILDING.md, BUILDING-cmake.md):

• BUILDING.md: fixed wrong Debian release/CMake-version note, removed the pre-4.0 autoconf paragraph, corrected the test-UI binary name and documented ENABLE_SDL_UI=ON (it defaults to OFF and is not installed), typo fixes.
• BUILDING-cmake.md: find_package(libprojectM) -> find_package(projectM4) to match the installed projectM4 config package; corrected option defaults (ENABLE_SDL_UI is OFF); documented missing options (ENABLE_SYSTEM_PROJECTM_EVAL, ENABLE_BOOST_FILESYSTEM, ENABLE_MACOS_FRAMEWORK, BUILD_DOCS); corrected ENABLE_INSTALL/ENABLE_EMSCRIPTEN semantics; VS 2019 -> VS 2022 examples to match CI; typo fix.

Sphinx docs (docs/):

• building.rst was a pre-4.0 Autotools/Qt5/libvisual guide; rewritten as a pointer to BUILDING.md/BUILDING-cmake.md plus a short quick start, so there is a single source of truth.
• Removed llvm.rst (LLVM JIT no longer exists in the build) and its toctree entry.
• conf.py: version 4.1.0 -> 4.2.0 to match CMake PROJECT_VERSION; Doxygen XML path overridable via PROJECTM_DOXYGEN_XML env var.
• Typo fixes in EMSCRIPTEN.md and docs/emscripten.rst (parameterrs, WenGL).

[kblaschke]

If possible, you could move all build docs to docs/building.rst (and also EMSCRIPTEN.md -> docs/emscripten.rst) instead and remove the Markdown files in the main dir. Since the generated Doxygen output isn't available anywhere yet, linking to the RST files in the main README.md should suffice.

This will keep only one instance of the docs, making it easier to update and reducing the chance of diverging from development.

GitHub renders RST fine (and it's also quite readable as-is), so for users visiting the repository, there shouldn't be much of a difference.

[thedavidweng]

If possible, you could move all build docs to docs/building.rst (and also EMSCRIPTEN.md -> docs/emscripten.rst) instead and remove the Markdown files in the main dir. Since the generated Doxygen output isn't available anywhere yet, linking to the RST files in the main README.md should suffice.

This will keep only one instance of the docs, making it easier to update and reducing the chance of diverging from development.

GitHub renders RST fine (and it's also quite readable as-is), so for users visiting the repository, there shouldn't be much of a difference.

Done