Update docs with new `/complete` event

## Description
The [docs](https://docs.altinn.studio/events/subscribe-to-events/developer-guides/process-app-events/#default-altinn-app-event-types) currently list all available application events. This list needs to be updated with an entry for the newly created event as per Altinn/altinn-studio#17408.

PS: The [numbered list](https://docs.altinn.studio/events/subscribe-to-events/developer-guides/process-app-events/#example-flow-instance-data-processing) on the same docs page needs some CSS formatting. Apply `{.floating-bullet-numbers}` or `{.floating-bullet-numbers-sibling-ol}` markdown helper to assist with this.

Open the linked process app events documentation page and locate the list of default Altinn app event types. Add an entry for the new `/complete` event as described in Altinn/altinn-studio#17408, then apply the `{.floating-bullet-numbers}` or `{.floating-bullet-numbers-sibling-ol}` helper to the numbered list and verify the rendered formatting.

Written by the indexing model from the issue text.