Utilities/Sphinx: Add index entries for cross-references

Add a document transform to insert index and target nodes just before
any CMake domain cross-reference node.  This will make references to
CMake domain objects appear in the index.  Also add a comment explaining
why it cannot be done in a result_nodes method of the CMakeXRefRole.
Merge topic 'doc-index-xrefs' into next

fb59b23a Utilities/Sphinx: Add index entries for cross-references
Help: Link to GNUInstallDirs from CMAKE_INSTALL_PREFIX (#15246)

Add a note to the CMAKE_INSTALL_PREFIX documentation that refers readers
to the GNUInstallDirs module to make the latter easier to discover.
Merge topic 'doc-CMAKE_INSTALL_PREFIX-link-GNUInstallDirs' into next

00d4cdc9 Help: Link to GNUInstallDirs from CMAKE_INSTALL_PREFIX (#15246)