fix: decode XML entities in generated rustdoc

[0rlych1kk4]

What changed

quick-xml emits escaped XML entities as Event::GeneralRef. The parser was rebuilding those references as literal strings such as & and <, which meant rustdoc escaped them again in the generated documentation.
This change resolves predefined XML entities while parsing descriptions, so generated docs contain the intended characters.

For example:

(Dev & 0xff) << 24

is now generated as:

(Dev & 0xff) << 24

Unknown entity references are left unchanged.

Tests

• added a small escaped_html.xml fixture
• added snapshot coverage for & and < inside a Markdown code span
• updated existing snapshots where > and -> are now decoded

Validation

cargo test -p mavlink-bindgen
cargo test -p mavlink-bindgen --test e2e_snapshots
cargo fmt --all -- --check
git diff --check

Closes #506 .

[pv42]

lgtm 👍