docs: add a WASI section to the WebAssembly page

[bartlomieju]

The WebAssembly reference page had no mention of WASI, so anyone searching the
docs for "WASI" found nothing. This adds a "WebAssembly System Interface
(WASI)" section covering what WASI is, Deno's support for both generations, and
how to run modules, with working examples.

Everything in the section was built and run while writing it (Deno 2.8.3):

• WASI Preview 1: compiled a Rust program with
  rustc --target wasm32-wasip1 and ran the resulting .wasm with the
  node:wasi module (the run.ts shown), confirming args and env reach the
  guest.
• preopens: verified that mapping a directory with preopens lets the
  guest read a file from it, and that Deno requires --allow-read and
  --allow-write for the mapped path (so the permissions note is accurate).
• WASI Preview 2: compiled with rustc --target wasm32-wasip2 (a component,
  not a plain module) and confirmed both
  deno run -A npm:@bytecodealliance/jco run hello.wasm runs it and
  jco transpile emits an importable ES module.

Note: node:wasi prints an experimental-feature warning, which the section
calls out. The old @std/wasi module is gone from JSR, so it is intentionally
not referenced.

Closes #3203.