docs: add Proof of Inclusion documentation

[itsyaasir]

Description of change

Adds Proof of Inclusion as the third Notarization Toolkit component in the repository documentation and aligns its Rust, Wasm, and example READMEs with the existing component structure.

The updated documentation:

• explains when to use Proof of Inclusion and how it differs from the on-chain components;
• documents proof construction, verification, trust boundaries, and the proof model;
• adds Mermaid sequence diagrams for construction and verification;
• aligns Package terminology across the root, Rust, and Wasm documentation;
• documents how to build and install the poi CLI from source, since pre-built binaries are not distributed; and
• updates Rust and Wasm example prerequisites, learning objectives, and resource links.

Links to any relevant issues

None.

Type of change

• Bug fix (a non-breaking change which fixes an issue)
• Enhancement (a non-breaking change which adds functionality)
• Breaking change or feature that changes existing behavior
• Documentation Fix

How the change has been tested

• cargo test -p poi-rs --doc
• cargo check -p poi-rs
• cargo check -p poi-rs --features cli
• cargo build --release -p poi-rs --features cli --bin poi
• ./target/release/poi --version
• git diff --check
• Verified relative Markdown links and reviewed the PoI documentation with the repository naming-conventions skill.

Change checklist

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own changes
• I have made corresponding changes to the documentation
• I have checked that the relevant documentation and build checks pass locally