Document the UI for editing the notebook's `Project.toml`

[ariguiba]

A UI for editing the notebook's Project.toml with compat entries. (#3218 in Pluto)

@fonsp do we need this?

[fonsp]

Yes! 🙌

[ariguiba]

Turns out there is something in the website about this:

@fonsp Is this enough?

PS: I uncovered another bug: When I use the search function to search for example "Project.toml" if there is any entry that is a href, it doesn't appear in the results. I opened an Issue here about it #89

[fonsp]

This issue is about documenting http://github.com/fonsp/Pluto.jl/pull/3218 , did you see the PR and try it in Pluto?