Enable tag generation for doxygen (rapidsai#672)

This PR configures doxygen to generate a tag file. The tag file will ultimately be used by `cudf` for cross-referencing documentation as described in rapidsai/cudf#5152.

Authors:
  - AJ Schmidt (@ajschmidt8)

Approvers:
  - Dillon Cullinan (@dillon-cullinan)
  - Mark Harris (@harrism)

URL: rapidsai#672

Author: ajschmidt8

doxygen/Doxyfile

@@ -2081,7 +2081,7 @@ TAGFILES               =
 # tag file that is based on the input files it reads. See section "Linking to
 # external documentation" for more information about the usage of tag files.
 
-GENERATE_TAGFILE       =
+GENERATE_TAGFILE       = html/rmm.tag
 
 # If the ALLEXTERNALS tag is set to YES, all external class will be listed in
 # the class index. If set to NO, only the inherited external classes will be