Refactor the frontend testing guide

Problem to solve

• What product or feature(s) affected? CE and EE
• What docs or doc section affected? Include links or paths. Frontend Testing Guide
• Is there a problem with a specific document, or a feature/process that's not addressed sufficiently in docs? The guide currently has a lot of Jest and Karma interspersed within the doc. Some topics are fragmented in several sections. There is a substantial amount of Jest content inside the Karma section. Since we are trying to move away from Karma, we should make the docs reflect this and make Jest the primary focus.

Further details

Audience: frontend testers (do we need to involve technical writers if it's a document intended for devs only?)

Who can address the issue

Frontend testers

cc @winh @pslaughter

Other links/references

From writing 2 MRs (gitlab-ce!30901 and gitlab-ce!30791) that improve the frontend testing guide, it wasn't super clear where I should put new content. Revamping this doc would also improve quality of life for other devs that will contribute to it.