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.
Help: Add 3.1 release note Emacs mode update
Merge topic 'emacs-mode-underscore-in-symbol' into next

cb80e513 Help: Add 3.1 release note Emacs mode update
Merge topic 'doc-index-xrefs' into next

7ca9a459 Utilities/Sphinx: Add index entries for cross-references