Missing section about Emscripten integration

The old docs were here: https://docs.conan.io/1/integrations/cross_platform/emscripten.html

These docs seems almost correct -
I actually seem to force Conan to work with (system) Emscripten, but don't know how to force Emscripten's cmake toolchain.

Changes for docs:
- change `[env]` to `[buildenv]`;
- use emsdk/3.1.50 from conan-center;
- actualize the references and step.

My profile which seems "to work" (platform tool is for using system cmake, so as the CC and CXX in the buildenv):
```
[settings]
arch=wasm
build_type=Release
os=Emscripten
compiler=clang
compiler.version=18
compiler.libcxx=libc++

[platform_tool_requires]
cmake/3.30

[buildenv]
CC=emcc
CXX=em++
```

Seems related to #2943