Add docs for Gecko devs

Devs don't necessarily know about thread registrations, how to add their own markers, etc. (Based on questions in Slack's#perf.)

We could have a page that lists what's possible, with at least a link to GeckoProfiler.h, and maybe some more useful information. (But we should probably avoid duplicating docs from our headers, so docs here don't get out of sync.)

There are already some docs at https://developer.mozilla.org/en-US/docs/Mozilla/Performance/Profiling_with_the_Built-in_Profiler ; if that's the official place for more Gecko-specific information, we should link there, and maybe add extra Gecko information there.
Alternatively, #1199 is about transferring this MDN information here.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-152)

Start by reading the existing MDN profiling page and GeckoProfiler.h, then compare issue #1199 before choosing the canonical documentation location. Cover thread registrations and custom markers without duplicating header documentation; done means Gecko developers can find the relevant guidance and links in the agreed location.

Written by the indexing model from the issue text.