docs: add documentation for parse, validate, and instantiate subcommands

[Divyansh200102]

Explanation

Adds documentation for the three new CLI subcommands (parse, validate, instantiate) introduced in WasmEdge/WasmEdge#4694. Also updates the main CLI page to list all available subcommands.

Related issue

Closes WasmEdge/WasmEdge#4688

What type of PR is this

/kind documentation

Proposed Changes

• Added parse.md — documents the wasmedge parse subcommand, which displays a WASM module's section details (types, imports, functions, globals, exports, code, custom sections).
• Added validate.md — documents the wasmedge validate subcommand, which checks a WASM module against the spec without executing it.
• Added instantiate.md — documents the wasmedge instantiate subcommand, which loads, validates, and instantiates a module (with multi-module linking support via --module).
• Updated cli.md — added a subcommands table linking to all five subcommand pages, and updated the USAGE block to reflect the new CLI structure.

Each new page follows the same structure and conventions as the existing subcommands documentations.

[Divyansh200102]

The following example .wat files are placeholders for now and will be added as examples in the main PR.

• parse_example.wat (used in parse.md)
• bad_validate.wat (used in validate.md)
• bad_instantiate.wat (used in instantiate.md)