Link to nested collections feature

[dylanirlbeck]

Summary

I spent too much time moving around folders/files in my project before realizing that the Nested Collections feature existed. I could have use the search function to find it listed under Beta Features, but I don't think that this should be a requirement. I add a section at the bottom of the "Folder Collections" section that discusses this important beta feature.

As a larger question, when do features move out of beta?

Test plan

N/A

Checklist

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[erezrokah]

@dylanirlbeck thank you for taking the time to submit this PR!
I'm sure others will find this useful

[erezrokah]

As a larger question, when do features move out of beta?

See #4608

For nested collections, I think we would need to provide a better way to edit the path as it's quite fragile at the moment.
See https://github.com/netlify-labs/netlify-cms-widget-parent as an experiment