add NrrdReader doc in data.rst (#6708)

### Description

add NrrdReader doc

### Types of changes
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [x] Non-breaking change (fix or new feature that would not break
existing functionality).
- [ ] Breaking change (fix or new feature that would cause existing
functionality to change).
- [ ] New tests added to cover the changes.
- [ ] Integration tests passed locally by running `./runtests.sh -f -u
--net --coverage`.
- [ ] Quick tests passed locally by running `./runtests.sh --quick
--unittests --disttests`.
- [ ] In-line docstrings updated.
- [ ] Documentation updated, tested `make html` command in the `docs/`
folder.

---------

Signed-off-by: HussainAlasmawi <91467092+HussainAlasmawi@users.noreply.github.com>

docs/source/data.rst

@@ -152,6 +152,10 @@ PILReader
 .. autoclass:: PILReader
   :members:
 
+NrrdReader
+~~~~~~~~~~
+.. autoclass:: NrrdReader
+  :members:
 
 Image writer
 ------------