docs: finalize the 0.1.0 documentation

[tisonkun]

Summary

• lead with the crate's purpose, direction selection, and graph model before detailed API material
• document the exact Serde metadata reflected by the derives and keep the three serde_shape extensions clearly separated
• align public rustdoc and Markdown tables with the documentation style used in datasketches-rust, including useful landing documentation for the derive crate
• remove the pre-release changelog and the contributor guidance that required maintaining it

This PR changes documentation only. It does not change public APIs or reflection behavior.

Review order

1. afd1d43 docs: explain the reflection contract before the API
   • Why: new users first need to decide between serialization and deserialization shapes and understand what the graph promises to preserve.
2. 071c9cb docs: align rustdoc with the public documentation style
   • Why: published API summaries should be consistent and the proc-macro crate should explain where its derives are normally consumed.
3. a6de9f9 docs: remove the pre-release changelog
   • Why: 0.1.0 is the first public release of the current design, so an internal history of unreleased rewrites is not useful release documentation.
4. 4413b1b docs: align Markdown table columns
   • Why: README tables should remain readable in source form as well as rendered output.

Documented design boundary

Serde attributes remain the source of truth for serialization behavior. The serde_shape namespace only fills reflection-specific gaps:

• serialize_with
• deserialize_with
• bound

It does not provide competing rename, tag, skip, flatten, alias, or default behavior.

Validation

• cargo x lint
• cargo x build --locked
• cargo x test
• cargo x package --locked
• cargo +1.85.0 x test
• cargo +1.85.0 check -p serde-shape --lib --no-default-features --target thumbv7em-none-eabihf
• cargo +1.85.0 check -p serde-shape-test-no-std --lib --target thumbv7em-none-eabihf