[libc][docs][NFC] Remove dead files and consolidate check.rst (#194442)
Deleted check.rst, Helpers/Styles.rst, dev/cmake_build_rules.rst, and
dev/clang_tidy_checks.rst. Moved the |check| substitution into
rst_prolog in conf.py so it is available globally without per-file
include directives.
Removed all '.. include:: check.rst' lines from hand-written header docs
and from the docgen.py generator that emits them for auto-generated
header pages.
Merged the clang-tidy checks documentation into code_style.rst under a
new 'Static Analysis & Clang-Tidy' section, preserving the
_clang_tidy_checks label for existing cross-references.
Updated code examples in both libc docs and the upstream clang-tidy
check docs to replace the stale LLVM_LIBC_ENTRYPOINT macro with the
current LLVM_LIBC_FUNCTION macro.
Updated dev/index.rst to drop the two deleted toctree entries.
llvm
a23ddcd3 - [libc][docs][NFC] Remove dead files and consolidate check.rst (#194442)
