[PATCH v8 0/2] docs: Integrate rustdoc into Rust documentation

[Carlos Bilbao <carlos.bilbao@amd.com>]

Include HTML output generated with rustdoc into the Linux kernel
documentation on Rust.

Carlos Bilbao:
 docs: Move rustdoc output, cross-reference it
 docs: Integrate rustdoc generation into htmldocs

---
V7 is https://lore.kernel.org/lkml/20230717151624.3470714-1-carlos.bilbao@amd.com/T/#m5dcab9c7775e85331c0d067d411fe246ed39af1e
Changes since V7:
 - Use lowercase for rustdoc_output and define it with `:=`.
 - Remove messages that would be printed when skipping Rust documentation.

V6 is https://lore.kernel.org/lkml/20230127165728.119507-1-carl...
Changes since V6:
 - Skip Rust document generation entirely if .config doesn't exist.

Changes since V5:
 - Continue executing htmldocs even if rustdoc fails.

Changes since V4:
 - Limit rustdoc note only to html outputs.

Changes since V3:
 - Added Reviewed-Bys from Akira Yokosawa.
 - PATCH 1/2: Avoid error 404 adding tag `rustdoc` for Sphinx.
 - PATCH 1/2: Don't use "here" as link text, describe destination instead.
 - PATCH 2/2: Check CONFIG_RUST in a way that allows us to skip generation.
 - PATCH 2/2: Reoder Sphinx runs so they complete even if rustdoc fails.

Changes since V2:
 - Split v2 into two-patch series.
 - Add "only:: html" directive in Documentation/rust/index.rst reference

Changes since V1:
 - Work on top of v6.1-rc1.
 - Don't use rustdoc.rst, instead add link to Documentation/rust/index.rst.
 - In Documentation/Makefile, replace @make rustdoc for $(Q)$(MAKE) rustdoc.
 - Don't do LLVM=1 for all rustdoc generation within `make htmldocs`.
 - Add spaces on definition of RUSTDOC_OUTPUT, for consistency.

---
 Documentation/Makefile       | 16 ++++++++++++++++
 Documentation/rust/index.rst |  8 ++++++++
 rust/Makefile                | 15 +++++++++------
 3 files changed, 33 insertions(+), 6 deletions(-)