add artifact preview (#533)

Document Actions artifact previews added in https://github.com/go-gitea/gitea/pull/36754, including supported content, authentication and sandboxing behavior, navigation, and size limits.

Also document the `ARTIFACT_PREVIEW_MAX_SIZE` configuration option so administrators can limit or disable previews.

Co-authored-by: silverwind <[email protected]>
Reviewed-on: https://gitea.com/gitea/docs/pulls/533
Reviewed-by: Lunny Xiao <[email protected]>
Reviewed-by: silverwind <[email protected]>
Co-authored-by: bircni <[email protected]>

versioned_docs/version-28/administration/config-cheat-sheet.md

@@ -1708,6 +1708,7 @@ PROXY_HOSTS = *.github.com
 - `ARTIFACT_RETENTION_DAYS`: **90**: Days to keep artifacts. Old artifacts will be deleted after this period. 0 means keep forever.
   Changes only apply to newly uploaded artifacts, existing ones keep the expiry stored when they were uploaded.
   Artifacts could have their own retention periods by setting the `retention-days` option in `actions/upload-artifact` step.
+- `ARTIFACT_PREVIEW_MAX_SIZE`: **10485760**: Maximum total artifact size in bytes that can be browsed or previewed in the web UI. Set to `0` to disable artifact previews or `-1` for no limit. Individual files are also limited by [`MAX_DISPLAY_FILE_SIZE`](#ui-ui).
 - `RUN_RETENTION_DAYS`: **400**: Days to keep completed runs. Old runs and everything under them will be deleted after this period. 0 means keep forever.
 - `ZOMBIE_TASK_TIMEOUT`: **10m**: Timeout to stop the task which have running status, but haven't been updated for a long time
 - `ENDLESS_TASK_TIMEOUT`: **3h**: Timeout to stop the tasks which have running status and continuous updates, but don't end for a long time