Add information about filtering epics in a Roadmap

Problem to solve

In !47884 (comment 450287246) @uhlexis said:

It may be nice to enrich the docs with more information about filtering and why it is important @msedlakjakubowski . I don't know if we want to mention it alleviating performance issues 😆 since that is not the user's fault- but giving some pointers and encouragement to filter could be helpful.

Proposal

Add a new section at the end of the Roadmap doc, for example, one or many out of:

• "Filtering epics" with explanation how it works
• "Filter epics in a roadmap" with how-to steps
• "Performance tips" with info how to make your roadmap perform better

EDIT:
The roadmap doc now has a sorting and filtering topic: https://docs.gitlab.com/ee/user/group/roadmap/#sort-and-filter-the-roadmap. What's missing is the performance improvements tips.

Who can address the issue

A groupproduct planning Engineer, or Engineering Manager working with @msedlakjakubowski