Add recipes to documentation for mechanisms to enable toolbar on demand

[alimony]

The default mechanism to enable the toolbar on internal hosts and DEBUG = True works for many people and the rest use a custom callback. But what are some common types of callbacks to enable the toolbar only when you specifically ask for it?

I've seen people do this by using an extra query parameter to the request, by checking a cookie, and some other variants. Would be nice to have a few good working examples in the documentation since this is how a lot of people want to use the debug toolbar, i.e. not on all requests but only on demand.

[jdufresne]

@alimony Thanks for the idea. Would you like to contribute a PR with this suggestion?

[alimony]

I'll try and think of something. I'd need to research it a bit so the suggestions are actually good practices.

[JulienPalard]

I did this a year ago and I'm very happy with it:

https://framagit.org/hackinscience/hkis-website/-/commit/bed20c487e9de9a69d8f87dd0e86eb050fe82876

It uses a cookie storing a secret, if the secret is here the toolbar is enabled else it is not. Then it use Django admin to provide a checkbox to set/unset the cookie:

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