misc: Added src/doxygen/html to .gitignore

misc: Added src/doxygen/html to .gitignore

Previously `src/doxygen` was ignored, but `src/doxygen` contains some
"source" for creating the doxygen html. Therefore this .gitignore entry
has been removed and replaced with one that only ignores the generated
`src/doxygen/html`.

Change-Id: I5add9fe839a00ad9d216d2082beda637ad0ea87d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28389
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>