Point File System docs to WasmFS tracking project instead of closed tracking issue

Progress on WasmFS is now tracked via [a project](https://github.com/orgs/emscripten-core/projects/1/views/1) instead of [an issue](https://github.com/emscripten-core/emscripten/issues/15041).

[The docs however still point to the issue](https://github.com/emscripten-core/emscripten/blob/bf5d63c2c0cf244a5a8c31f2261c0d01c49d8f7b/site/source/docs/api_reference/Filesystem-API.rst?plain=1#L34), in which [the link to the project](https://github.com/emscripten-core/emscripten/issues/15041#issuecomment-1325556023) is hidden inside the comments.