The mling CLI tool is being rewritten, so remove all its source files,
resources, and workspace membership to clean up the repository

Add `#![deny(missing_docs)]` across multiple crates and fill in
documentation for all public items, including struct fields, enum
variants, trait methods, and proc macros.  Also mark two shell scripts
as executable.

Use Option for load_pathf_map return type and emit a descriptive
compile_error when the type mapping file cannot be loaded, guiding users
to set up build.rs correctly

- Move PickerArg-to-PickerArgInfo conversion into an `into_info` method
- Add `short_flag()` and `long_flag()` methods to PickerArgInfo

Add `docs_rs` feature flag to enable docs.rs-specific configuration,
including `--generate-link-to-definition` and logo display.

Include the feature in build scripts, editor settings, and feature
constants for consistent documentation generation.

Use `--manifest-path mingling/Cargo.toml` instead of `--workspace` to
avoid generating documentation for unintended root workspace crates

Add a helper script `.run/src/bin/cov-test.rs` that generates
HTML coverage reports using `cargo llvm-cov` and moves the
output files to the `docs/cov-test/` directory