PGD: add documentation section considering extension usage

[ibarwick]

What Changed?

The subject of installing other extensions in a PGD cluster has come up a couple of times recently, but the documentation doesn't really say much about it, so add a section discussing various relevant issues etc., and also explicitly list the extensions which are known to be compatible.

BDR-5141.

[gvasquezvargas]

@ibarwick it looks like DJ dropped some suggestions for you to take a look. Could you take look and integrate them? We can take care of merging afterwards.

[ibarwick]

@gvasquezvargas apologies for the delay, this got a little lost somewhere.

I've refactored it based on DJ's suggestions and submitted it as ready-for-review.

The PR is against the develop branch; there is nothing specific to PGD 5.6.

[github-actions[bot]]

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