docs: move detailed README documentation into topic guides

[egorsmkv]

Closes #20.

The root README is now a compact project page with features, installation, the unchanged complete QuotesSpider quickstart, and documentation navigation. Installation and feature details live in docs/installation.md; request/selector examples, runtime configuration, middleware, pipelines, runnable examples and benchmarks are consolidated into the existing topic guides. Existing utility API and logging guides replace duplicated README material, and the documentation index and relative links are updated.

The complete README quickstart remains byte-for-byte unchanged, so scripts/check_docs.py continues to compile and execute its local integration fixtures. Only Markdown files change; library behavior and CI tooling are unchanged.

Validation:

• All local Markdown destinations, heading anchors and code fences pass scripts/check_docs.py.
• All 25 Rust snippets compile on Rust 1.92 in default, xpath/cli-examples, and no-default-feature configurations; the local quickstart and parsing/pagination tests pass in each configuration.
• cargo +1.92.0 test --locked --doc --features xpath,cli-examples passes (the crate currently contains zero rustdoc tests).
• RUSTDOCFLAGS="-D warnings" cargo doc --locked --no-deps --features xpath,cli-examples passes on Rust 1.99.
• cargo doc on Rust 1.92 hits an internal rustdoc assertion in html/render/search_index.rs; no Rust source was changed. This toolchain limitation remains visible for review.
• External Markdown links were checked: GitHub, docs.rs, PyPI and badge URLs respond successfully. Six crates.io crate pages return HTTP 404 in this environment, so their availability could not be verified; their existing canonical URLs are retained.
• git diff --check passes.