Update SCE contents and processing with cell type information

[allyhawkins]

Closes #160

This PR makes some small adjustments to docs to sync up what we say about cell typing here to where the cell types actually live in the object.

• I added a link for both SingleR and CellAssign papers to the processing information section on cell type annotation.
• In that same section, I added a note about submitter annotations.
• I included the submitter_annotation column in the cell metrics section. When adding this in, I chose to add this in the table that mentions items that are in both filtered and processed objects. I think this is the right place since if they are available they will be in all of the objects.
• I standardized how we refer to the cellassign_reference and singler_reference. This matches changes made in https://github.com/AlexsLemonade/scpca-nf/pull/510 and https://github.com/AlexsLemonade/scpca-nf/pull/503.

Please let me know if you think I missed any other areas here. I will update the download section separately as part of #159 and the getting started section as part of #165.

[sjspielman]

@allyhawkins FYI I'm going to get to this today but realize I forgot it in my scrum!

[allyhawkins]

Thanks for taking a look at this @sjspielman. I incorporated your comments and then also filed https://github.com/AlexsLemonade/scpca-nf/pull/522 to return the label name. I then updated the docs here to reflect that.

[sjspielman]

for posterity noting that singler_reference_label metadata item referred to here matches the name used in https://github.com/AlexsLemonade/scpca-nf/pull/522 👍