Add rust_workspace_doc for merged workspace documentation (#1837)

[havasd]

Adds a rust_workspace_doc rule that documents every workspace crate transitively reachable from its deps and merges the results into a single documentation tree with a unified search index, like cargo doc in a Cargo workspace.

The implementation uses the rustdoc cross-crate merge flags from RFC 3662 (--merge, --parts-out-dir, --include-parts-dir), which are nightly-only until rust-lang/rust#130676 stabilizes:

• A rust_workspace_doc_aspect walks the dependency graph and runs one independently cached rustdoc --merge=none action per crate, producing an HTML directory and a cross-crate-info parts directory.
• A finalize action documents a stub crate with --merge=finalize to produce the merged search index and (by default) a root index page listing all crates (--enable-index-page).
• A new doc_merger helper assembles the final tree, exported both as a directory and as a <name>.zip suitable for archiving.

A tools/rustdoc_workspace:gen_workspace_docs binary offers a rule-free alternative in the style of gen_rust_project: it applies the aspect to target patterns, discovers the per-crate outputs from the build event stream (including under remote execution, where output paths are reconstructed from BEP pathPrefix entries and downloads are forced with --remote_download_regex), runs the finalize step and assembles the tree into a chosen output directory.

Also:

• Binary crates are documented like cargo doc --bins: their outputs use a distinct _bin directory suffix and a binary whose crate name collides with another documented crate yields to it. Colliding library crates remain an analysis error.
• process_wrapper gains a repeated --mkdir flag, used to pre-create dependency directories in each crate's out-dir so rustdoc emits relative cross-crate links into the merged tree.
• rustdoc_compile_action accepts attr/file/files overrides so aspects can reuse it, and tolerates synthetic crates without output.
• External crates are skipped by default; the new //rust/settings:rustdoc_workspace_include_external flag opts in.
• The repeatable //rust/settings:rustdoc_workspace_extra_flag setting adds flags (e.g. -Dwarnings, --document-private-items) to every per-crate rustdoc invocation.
• On non-nightly toolchains the rule fails with a descriptive error and the aspect produces no outputs, so example targets can be gated with target_compatible_with on the toolchain channel.