Replace docs `PipelineCachedStorage` references with `PipelineManifestStorage`

[Archmonger]

PipelineCachedStorage is gone from django 3.1+, and the docs should be updated to reflect that.

[kevinmarsh]

There's an open PR (from a year ago 😬 ), does that do a good enough job addressing this deprecation?

[Archmonger]

It would be best to also mention PipelineManifestStorage should be used instead, in my opinion.

[Pyvonix]

I just merge the PR for the doc.
Should you think we really need to completely replace it in the doc?

[Archmonger]

The current changes are fine with me.

But, it doesn't look like the docs were ever rebuilt after merging that PR. This issue should remain open until a docs rebuild.