DOC: Mark steps as optional in flowchart

[larsoner]

@larsoner is the 'temporal regression for artifact removal' a default step? I am not aware that I am using it.

If not, I think it should be marked as optional.

Originally posted by @SophieHerbst in #860 (comment)

[larsoner]

In some sense almost all steps are optional. Like if you set l_freq = None (default) and h_freq = None (not default!) there is no frequency filtering, spatial_filter = None (default) skips SSP and ICA, etc. I'll add a note to #865 along these lines. After that I think that documenting better which config options affect which steps is probably the best we can do (?), in which case this would be a dup of #862