Fix: Docs generation create temporary files that are not cleaned up.

[andrewendlinger]

When building the documentation, a couple of example files are written to disk but never deleted.

Currently affected:

• doc/user-guide/io.rst

  • In line 608 saved_on_disk.h5 is written to disk but never removed
    (note: .h5 files are not exlcuded by .gitignore)

• doc/internals/time-coding.rst

  • test-datetimes1.nc
  • test-datetimes2.nc
  • test-timedeltas1.nc
  • test-timedeltas2.nc

This PR adds a simple clean up snippet at the end of each affected file / section to get rid of them:

.. ipython:: python
    :suppress:

    # Cleanup
    import os

    for f in ["i_am_a_tmp_example.nc", "tmp_file.h5"]:
        if os.path.exists(f):
            os.remove(f)

[dcherian]

thanks!