DOC: Relevant API links for intersphinx mapping (#1931)

* DOC: added API links to intersphinx mapping

* DOC: updated mergingdata.rst links

* DOC: updated aggregation_with_dissolve.rst links

* DOC: updated data_structures.rst links

* DOC: updated geocoding.rst links

* DOC: updated geometric_manipulations.rst links

* DOC: updated indexing.rst links

* DOC: updated io.rst links

* DOC: updated projections.rst links

* DOC: updated set_operations.rst links

* DOC: updated mapping.rst links

* DOC: updated missing_empty.rst links

* DOC: updated geoplot intersphinx links

* DOC: make 'unary_union' attr instead of method

Co-authored-by: Martin Fleischmann <martin@martinfleischmann.net>

* DOC: remove link to GeoDataFrame.geometry

Co-authored-by: Martin Fleischmann <martin@martinfleischmann.net>

* DOC: convert pandas indexers to attrs instead of methods

Co-authored-by: Martin Fleischmann <martin@martinfleischmann.net>

* DOC: convert indexer 'cx' to attr instead of method

Co-authored-by: Martin Fleischmann <martin@martinfleischmann.net>

* DOC: refer GeoSeries.buffer instead of shapely buffer

Co-authored-by: Martin Fleischmann <martin@martinfleischmann.net>

* DOC: make 'unary_union' attr instead of method on missing_empty.rst

Co-authored-by: Martin Fleischmann <martin@martinfleischmann.net>

* DOC: refer to DataFrame.merge instead of pandas.merge

Co-authored-by: Martin Fleischmann <martin@martinfleischmann.net>

* DOC: link pyproj.CRS

* DOC: changed API links to 'stable' from 'latest'

* DOC: fixed separator length

* DOC: uppercase CRS in pyproj.crs

Co-authored-by: Martin Fleischmann <martin@martinfleischmann.net>

doc/source/docs/user_guide/missing_empty.rst

@@ -22,7 +22,7 @@ empty geometries:
   a Shapely geometry object.
 - **Missing geometries** are unknown values in a GeoSeries. They will typically
   be propagated in operations (for example in calculations of the area or of
-  the intersection), or ignored in reductions such as ``unary_union``.
+  the intersection), or ignored in reductions such as :attr:`~GeoSeries.unary_union`.
   The scalar object (when accessing a single element of a GeoSeries) is the
   Python ``None`` object.