Improve documentation on secrets management of editor extensions for GitLab Duo
<!--IssueSummary start-->
<details>
<summary>
Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards.
</summary>
- [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=509067)
</details>
<!--IssueSummary end-->
<!-- This template is a great use for issues that are feature::additions or technical tasks for larger issues.-->
### Proposal
<!-- Use this section to explain the feature and how it will work. It can be helpful to add technical details, design proposals, and links to related epics or issues. -->
<!-- Consider adding related issues and epics to this issue. You can also reference the Feature Proposal Template (https://gitlab.com/gitlab-org/gitlab/-/blob/master/.gitlab/issue_templates/Feature%20proposal%20-%20detailed.md) for additional details to consider adding to this issue. Additionally, as a data oriented organization, when your feature exits planning breakdown, consider adding the `What does success look like, and how can we measure that?` section.
-->
A ~"GitLab Dedicated" ~customer ([ZD ticket, internal only](https://gitlab.zendesk.com/agent/tickets/588032)) is assessing GitLab Duo Enterprise and have questions regarding secrets management (how is it **stored** and **encrypted**) for all editor extensions with GitLab Duo.
Using this issue to gather the answers to eventually update the documentation.
<!-- Label reminders
Use the following resources to find the appropriate labels:
- Use only one tier label choosing the lowest tier this is intended for
- https://gitlab.com/gitlab-org/gitlab/-/labels
- https://about.gitlab.com/handbook/product/categories/features/
-->
issue