chore: Extract JSON computation and caching into a separate crate

[LukeMathWalker]

Extract the JSON documentation caching logic from pavexc into a new crate pavexc_rustdoc_cache.

The rustdoc cache was invalidated on every git commit because the database filename included the git SHA ({VERSION}-{GIT_SHA}.db). This was problematic during local development: each commit wiped the cache, forcing expensive re-computation of JSON docs.

The cache fingerprint is now {VERSION}-{SOURCE_HASH} where:

• VERSION is pavexc_rustdoc_cache's crate version
• SOURCE_HASH is an xxhash of the source files in pavexc_rustdoc_cache and its local dependencies {pavexc_attr_parser, pavex_bp_schema)

The hash is computed in pavexc/build.rs by automatically discovering path dependencies from Cargo.toml.