GH-49789: [C++] Use CMAKE_INSTALL_DOCDIR instead of static share/doc/${PROJECT_NAME}

[Swaroop883]

Rationale for this change

ARROW_DOC_DIR was hardcoded as "share/doc/${PROJECT_NAME}"
which does not respect system-specific documentation directory
conventions. All other install directories in this file already
use GNUInstallDirs variables (e.g. CMAKE_INSTALL_DATADIR,
CMAKE_INSTALL_LIBDIR). This change makes ARROW_DOC_DIR
consistent with that pattern.

What changes are included in this PR?

• Replaced set(ARROW_DOC_DIR "share/doc/${PROJECT_NAME}") with
  set(ARROW_DOC_DIR "${CMAKE_INSTALL_DOCDIR}") in
  cpp/CMakeLists.txt.

Are these changes tested?

No new tests are needed. This is a build system change that affects
only the install path of documentation files (LICENSE.txt, NOTICE.txt,
README.md). The behavior is identical on standard systems where
CMAKE_INSTALL_DOCDIR defaults to share/doc/${PROJECT_NAME}.

Are there any user-facing changes?

No. On standard Linux/macOS systems the install path remains
share/doc/arrow. Users who explicitly override CMAKE_INSTALL_DOCDIR
will now have their setting respected, which is the correct behavior.

• GitHub Issue: [C++] Use CMAKE_INSTALL_DOCDIR instead of static share/doc/${PROJECT_NAME} #49789

[Swaroop883]

Closing this as the fix has already been merged in #49790.
Thanks for the quick response!