sig create/update --soft-delete option documentation

While investigating the options for preventing accidental loss of images through deletion, I noticed the `--soft-delete` option for `az sig create` and `az sig update`. The current explanation in the docs stating it enables soft-deletion with a retention period allowing recovery of items in the gallery seems to fit my needs. However I cannot find the necesarry related documentation on:

- The default retention period
- Configurability of the retention period
- How to restore soft-deleted items
- Generic description of the feature on the Compute Gallery service overview page.

---
#### Document Details

⚠ *Do not edit this section. It is required for learn.microsoft.com ➟ GitHub issue linking.*

* ID: b1f9155c-108c-b602-3d8e-d17a213eb3d0
* Version Independent ID: 90421467-30f0-75b6-d171-02b8e37e7d07
* Content: [az sig](https://learn.microsoft.com/en-us/cli/azure/sig?view=azure-cli-latest)
* Content Source: [latest/docs-ref-autogen/sig.yml](https://github.com/MicrosoftDocs/azure-docs-cli/blob/main/latest/docs-ref-autogen/sig.yml)
* GitHub Login: @rloutlaw
* Microsoft Alias: **routlaw**