Add guide for extending product list search filters (OCC-260)

[sarahelsaig]

Is your feature request related to a problem? Please describe

Product lists are briefly mentioned in the Creating a simple webshop guide, but not enough info is provided to customize it, and even the built-intitle search is not mentioned.

Describe the solution you'd like

It would be good to add an article in the guides section of the docs. It should describe what files need to be added and how, to include an additional search field.

Also, update the screenshot above this line to show the built-in filter.

Jira issue

[sarahelsaig]

One of our client projects includes a custom category filter, that can be used for reference when writing the doc.