Add more documentation explaining how to debug specific scenarios

We already have such documentation in https://profiler.firefox.com/docs/#/./case-studies, but it would be good to have some simpler ones.

Here are some documentation from the devtools older panel: https://hg.mozilla.org/mozilla-central/file/0242545b34ca3f3290c68496c2e921ddfdf5cdc3/devtools/docs/user/performance/scenarios and https://hg.mozilla.org/mozilla-central/file/0242545b34ca3f3290c68496c2e921ddfdf5cdc3/devtools/docs/user/performance/examples

Here are the scenarios that were described: https://github.com/firefox-devtools/performance-scenarios

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-554)