Better documentation of editing omni.ja

Proposal:
Editing the omni.ja file - e.g. just by opening it in 7zip - is a really simple way of trying changes to the Thunderbird JavaScript source without setting up any development environment. That however requires a consecutive launch of Thunderbird using the --purgecaches flag or using the MOZ_PURGE_CACHES environment variable, otherwise cached JS code instead of the custom one is used. This is such a simple way to start with Thunderbird development that I don't really understand why it isn't really documented anywhere official. Therefore I'm proposing that this is documented a bit more prominently.

The same probably goes for Firefox.

The current situation:
There exists https://wiki.mozilla.org/Thunderbird:Start_Hacking which I think should be integrated into and forward to a subpage of https://developer.thunderbird.net/thunderbird-development/getting-started . At one point something similar already existed in MDN, since the wiki page was to a significant extent duplicated by https://web.archive.org/web/20210620190432/https://developer.mozilla.org/en-US/docs/Mozilla/About_omni.ja_(formerly_omni.jar) which has been removed since then.
None of these sources documents the --purgecaches flag or the MOZ_PURGE_CACHES environment variable. The wiki page even mentions removing jsloader and jssubloader directories which I think is an outdated, discontinued alternative to clear the cache.

Context:
This is a follow-up to https://phabricator.services.mozilla.com/D185644#6141303 where this knowledge was quite useful.

Start with the Thunderbird:Start Hacking wiki page and the developer.thunderbird.net Thunderbird development getting-started page. Document editing omni.ja, launching with --purgecaches or MOZ_PURGE_CACHES, and clarify the outdated cache-clearing guidance; integrate or forward the wiki content as proposed. Done means the official getting-started path prominently covers this workflow.