Docs/wasm function support

[SurbhiAgarwal1]

Description

WASM functions are supported in kpt but there's no documentation on how to run, develop, or deploy them. This PR adds a comprehensive guide covering the complete WASM function workflow.

Motivation

Users need documentation to understand:

• How to run WASM functions with the --allow-alpha-wasm flag
• How to publish WASM modules using kpt alpha wasm push/pull
• How to develop WASM functions with proper build tags
• The benefits and limitations of WASM functions vs container-based functions

Without this documentation, users have to dig through code or CLI help to figure out WASM support.

Changes

Added documentation/content/en/book/04-using-functions/wasm-functions.md covering:

• Running WASM functions with fn render and fn eval
• Publishing and pulling WASM modules to/from OCI registries
• Developing Go-based WASM functions with complete code examples
• Benefits (faster startup, smaller size, better security)
• Limitations (alpha status, sandboxed execution, compatibility)

The code examples are based on actual WASM functions in krm-functions-catalog (set-namespace, set-labels, starlark) and follow the same pattern with separate build tags for regular and WASM builds.

Fixes #4296