Document gotchas with sources generated in actions during sync_release

[nforro]

Let's say that a project generates additional sources in post-upstream-clone or pre-sync action and then synces them using files_to_sync so they are picked up and uploaded to lookaside cache. If sync_release runs for multiple branches, unless the generated files are identical in each run (i.e. have the same hash), they are uploaded to lookaside cache every time, wasting resources.
One way to prevent that, other than ensuring that source generation is reproducible (which could be hard or even impossible to do), would be to check if the file is already present in lookaside cache before generating it.

Document this, likely here:

packit.dev/docs/configuration/examples.md

Lines 110 to 131 in b39979c

	### Custom archive creation for release syncing
	If you need to generate local archive(s) during release syncing, you can utilise e.g. `pre-sync` action
	to place the commands necessary for the creation. You also have to include the archive(s) in the list of files to
	be moved to the dist-git repo so that it is then handled by Packit from there.
	<details>
	<summary>Creating an archive using tito and syncing it to dist-git repo</summary>
	```yaml
	actions:
	pre-sync:
	- tito build -o . --tgz
	files_to_sync:
	- src:
	- "<my-package>-*.tar.gz"
	dest: .
	```
	</details>