Update docs to account for changes in AnnData objects for scanpy compatibility

[allyhawkins]

Based on the changes made in https://github.com/AlexsLemonade/scpca-nf/pull/774, we will need to make a few docs updates:

• X_PCA and X_UMAP are now lowercase X_pca and X_umap
• .var now has the highly_variable column
• .uns now includes pca metadata

The AnnData components section will need to be updated: https://scpca.readthedocs.io/en/stable/sce_file_contents.html#components-of-an-anndata-object

And the merged AnnData components section: https://scpca.readthedocs.io/en/stable/merged_objects.html#components-of-an-anndata-merged-object

[allyhawkins]

Closed by #338