fix: simplify crate landing docs

[f0rr0]

Summary

• reduce README to a simple landing page focused on use cases, install, and happy paths
• keep the main install path to pglite-oxide plus serde_json; anyhow is not required
• keep the SQLx compatibility example with its own optional SQLx/Tokio setup
• move detailed API, runtime/performance, and development material into separate docs
• keep public docs links absolute so they work from package renderers as well as GitHub

Verification

• cargo test --doc --locked
• cargo package --locked --allow-dirty --list includes README and the public docs
• cargo metadata --no-deps --format-version 1 confirms readme = "README.md" and docs.rs metadata
• git diff --check

Release note

This uses a fix: title intentionally: the README is part of the published crate package, so the simplified public docs need a patch release to become visible on package pages.