[Documentation] Lost link "Advanced Connection Management"
Hi,
I was reading the documentation about the `connection pool` and I think I have found a missing reference caused by a reorganization of documentation with the new versions.

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With the doc of the current version (v3.6.0), there is an error about an old link that is pointing to something that is not anymore in the structure documentation.

I'm speaking of the actual section of `Connection Pool` into the chapter `Playhous, extensions to Peewee`: at the end of the section missing a link:

<img width="1901" alt="connPoll-latest" src="https://user-images.githubusercontent.com/70535738/111475819-c434ba00-872d-11eb-9908-250952a9de6a.png">

In the `doc of the version 2.10.2` it was pointing to this section:

<img width="1917" alt="old-AdvConnMng" src="https://user-images.githubusercontent.com/70535738/111475973-efb7a480-872d-11eb-8e39-2410a0f433c1.png">

Actually, the above section doesn't exist anymore because, I think, it was move and update into this new section (actually present in the latest documentation):

<img width="1919" alt="new-AdvConnMng" src="https://user-images.githubusercontent.com/70535738/111476275-4c1ac400-872e-11eb-86db-b55dcbb1a6a4.png">

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I hope this could be useful.

Regards,
Ale