docs: add C integration guide

[JingsongLi]

What changed

• add a C Integration guide covering source builds, header generation, linking, catalog access, scan planning, and Arrow streaming reads
• document projection, predicate, write/commit, error, and resource ownership conventions for the C ABI
• add the guide to the MkDocs navigation
• update Cargo.lock to record the existing log dependency required by crates/paimon

Why

The repository contains a complete paimon-c ABI, but the documentation site did not explain how C consumers can build or use it. The lockfile was also missing the already-declared log dependency, causing every Rust CI command using --locked to fail before compilation.

Validation

• mkdocs build --strict
• generated paimon.h with cbindgen
• compiled the complete C example with cc -std=c11 -Wall -Wextra -Werror -fsyntax-only
• cargo +1.91.0 metadata --locked --format-version 1 --no-deps
• cargo +1.91.0 check --locked --workspace --all-targets --all-features