FAQ.md: jetstream needs to be backed up too. (#3327)

In the section "What data needs to be kept if transferring/backing up
Dendrite?" of the FAQ, add jetstream directory to the list.

It seems to be a common mistake when moving dendrite to a different
computer, that the jetstream directory is not copied.

### Pull Request Checklist

<!-- Please read
https://matrix-org.github.io/dendrite/development/contributing before
submitting your pull request -->

* [ ] I have added Go unit tests or [Complement integration
tests](https://github.com/matrix-org/complement) for this PR _or_ I have
justified why this PR doesn't need tests
* [x] Pull request includes a [sign off below using a legally
identifiable
name](https://matrix-org.github.io/dendrite/development/contributing#sign-off)
_or_ I have already signed off privately

Signed-off-by: `Anton Molyboha <[email protected]>`

docs/FAQ.md

@@ -117,6 +117,7 @@ The list of files that need to be stored is:
 - matrix-key.pem
 - dendrite.yaml
 - the postgres or sqlite DB
+- the jetstream directory
 - the media store
 - the search index (although this can be regenerated)