Implement a two-way mirroring between the wiki and its public repo
Previously, changes to the wiki would get merged to the public repo in a
once-a-week action. This significantly revises this, making the two
sides be mirrors (up to the few seconds it takes to do a merge).
This is driven by a minimal-ish yaml file in both sides (`TypeScript`
and `TypeScript-wiki`) that *always* works from the script in the public
repo.
The two action specs are nearly identical, but there are some differences:
- On the main repo, trigger on a `gollum` event, and in the wiki repo
the usual (pushes, schedule, manual). (The schedule run is kept as
a just-in-case, and it's now running twice a week.)
- The filename is `sync-wiki` on the TS side and just `sync` in the
wiki. (Good to avoid confusion if both files somehow find
themselves in the same neighborhood.)
- The secret names are different since I used the name that already
exists in each side.
The script does *not* start with a checkout of its repository. Doing
this in the TS side would be redundant (it would get the TS tree) and
slow. Instead, it's always cloning the public wiki repo (`DASHREMOTE`,
since its url is `.../TypeScript-wiki`) and then fetching into it the
repo of the rendered wiki (`DOTREMOTE`, with a `.../TypeScript.wiki`)
url.
Also revised the README, since they should always be mirrored with this
change, and therefore there is no "source of truth".
