[FIX][Rust] Let docs.rs build tvm-ffi and correct the Rust setup docs

[Seven-Streams]

The docs.rs build of tvm-ffi 0.1.14 failed
(https://docs.rs/crate/tvm-ffi/0.1.14/builds/4724950): rust/tvm-ffi/build.rs
runs tvm-ffi-config unconditionally and panics when it is missing, which is
the case on docs.rs and in docs/conf.py, which sets DOCS_RS the same way.
Bring the script in line with rust/tvm-ffi-sys/build.rs (#825):

• Under DOCS_RS, warn and skip the loader-path setup instead of panicking.
• Treat a non-zero exit status or empty output of tvm-ffi-config --libdir as
  an error, and report the exit status and stderr.
• Fail with the same install guidance as tvm-ffi-sys when DOCS_RS is not
  set.
• Rerun the script when DOCS_RS, PATH, the loader-path variable, or
  build.rs changes, so switching the Python environment updates the
  LD_LIBRARY_PATH baked into cargo run and cargo test instead of keeping
  the previous library directory.

Also fix two Rust documentation issues:

• rust/README.md claimed the build sets the dynamic loader path for
  cargo run and cargo test. That only holds inside this workspace: Cargo
  applies rustc-env from the running package's own build script and drops
  rustc-link-search paths outside the target directory, so downstream crates
  must set LD_LIBRARY_PATH themselves, including for cargo run and
  cargo test.
• docs/guides/rust_lang_guide.md still pointed published users at
  tvm-ffi = "0.1.0-alpha.0"; use 0.1.14.

Validation: with no tvm-ffi-config on PATH, DOCS_RS=1 cargo doc --no-deps -p tvm-ffi now succeeds with a warning (it previously panicked at build.rs:69)
and a plain cargo build -p tvm-ffi still fails, now with the install
guidance. With the Python package installed, cargo test -p tvm-ffi --test test_function and cargo run --example load_library --features example pass;
switching PATH between two environments reruns the build script and bakes the
new library directory, and an unchanged PATH does not rerun it. pre-commit
passed on all files.