Document CLI and library usage

[Drew-Chase]

I originally wrote this as a git submodule for the wiki, until I realized that you couldn't create a pull request for the wiki repo, so I've put it in the docs directory as a compromise.

• Add comprehensive documentation for pdu CLI usage.
• Add and refine wiki documentation for the reusable Rust library.
• Document core library concepts, including tree building, data transformation, rendering, sizing, JSON workflows, hardlinks, reporters, and feature flags.

Notes

This PR focuses on improving user-facing and contributor-facing documentation. It makes the CLI and library APIs easier to discover, explains common workflows, and consolidates the wiki documentation without keeping the wiki as a Git submodule.

Note

The docs were written by a human and checked by AI

[KSXGitHub]

Thanks for taking the time to write these docs.

I am not oppose to this idea by itself. However, free unbound markdown files risk stale.

I rather you add them to the source code.

If you still need markdown files for some reason, then you may write a subprogram (as a [[bin]] in Cargo.toml) to generate the markdown files from source.

Ideally, all the code and the identifiers are checked by either the compiler or the tests.

The more constrains can be encoded and tracked by automation, the better.