Make codegen I/O-free and agnostic to output location (#3888)

**Commit by commit**

This is necessary refactoring work for the upcoming
`attr.rust.custom_crate` attribute, itself necessary for the upcoming
serde-codegen support, itself necessary for the upcoming blueprint
experimentations as well as #3741.

### Changes

1. The `CodeGenerator` trait as well as all post-processing helpers
(gitattributes, orphan detection...) are now I/O-free.
  ```rust
pub type GeneratedFiles =
std::collections::BTreeMap<camino::Utf8PathBuf, String>;
  
  pub trait CodeGenerator {
      fn generate(
          &mut self,
          reporter: &crate::Reporter,
          objects: &crate::Objects,
          arrow_registry: &crate::ArrowRegistry,
      ) -> GeneratedFiles;
  }
  ```
2. All post-processing helpers are now agnostic to the location output.
This is very important as it makes it possible to generate e.g. rust
code out of the `re_types` crate without everything crumbling down.
A side-effect is that gitattributes files are now finer-grained.
3. The Rust codegen pass is now crate agnostic: it is driven by the
workspace path rather than a specific crate path.
Necessary for the upcoming `attr.rust.custom_crate`.
4. All codegen passes now follow the exact same 4-step structure:
  ```
  // 1. Generate in-memory code files.
  let mut gen = MyGenerator::new();
  let mut files = gen.generate(reporter, objects, arrow_registry);
  // 2. Generate in-memory attribute files.
  generate_gitattributes_for_generated_files(&mut files);
  // 3. Write all in-memory files to disk.
  write_files(&gen.pkg_path, &gen.testing_pkg_path, &files);
  // 4. Remove orphaned files.
  crate::codegen::common::remove_orphaned_files(reporter, &files);
  ```
5. The documentation codegen pass now removes its orphans, which is why
some `md` files were removed in this PR.

---

- Unblocks #3741 
- Unblocks #3495

crates/re_types_builder/src/bin/build_re_types.rs

@@ -15,7 +15,6 @@ const DEFINITIONS_DIR_PATH: &str = "crates/re_types/definitions";
 const ENTRYPOINT_PATH: &str = "crates/re_types/definitions/rerun/archetypes.fbs";
 const DOC_EXAMPLES_DIR_PATH: &str = "docs/code-examples";
 const CPP_OUTPUT_DIR_PATH: &str = "rerun_cpp";
-const RUST_OUTPUT_DIR_PATH: &str = "crates/re_types/.";
 const PYTHON_OUTPUT_DIR_PATH: &str = "rerun_py/rerun_sdk/rerun";
 const PYTHON_TESTING_OUTPUT_DIR_PATH: &str = "rerun_py/tests/test_types";
 const DOCS_CONTENT_DIR_PATH: &str = "docs/content/reference/types";
@@ -72,7 +71,6 @@ fn main() {
     let definitions_dir_path = workspace_dir.join(DEFINITIONS_DIR_PATH);
     let entrypoint_path = workspace_dir.join(ENTRYPOINT_PATH);
     let cpp_output_dir_path = workspace_dir.join(CPP_OUTPUT_DIR_PATH);
-    let rust_output_dir_path = workspace_dir.join(RUST_OUTPUT_DIR_PATH);
     let python_output_dir_path = workspace_dir.join(PYTHON_OUTPUT_DIR_PATH);
     let python_testing_output_dir_path = workspace_dir.join(PYTHON_TESTING_OUTPUT_DIR_PATH);
     let docs_content_dir_path = workspace_dir.join(DOCS_CONTENT_DIR_PATH);
@@ -120,7 +118,7 @@ fn main() {
         ),
         || re_types_builder::generate_rust_code(
             &reporter,
-            rust_output_dir_path,
+            workspace_dir,
             &objects,
             &arrow_registry
         ),