chore: rename manager/member to administrator (#11542)

# Which Problems Are Solved

As part of the naming consistency initiative, the terms `Manager`,
`Member` and other similar terms that are used in the context of an
administrator are replaced by `Administrator` in the UI and
documentation.

# How the Problems Are Solved

By replacing the following terms with Administrator / Administrator
Roles in docs, Console UI, and guides.

- Manager
- Add Manager
- Add a Manager
- Administrator (used as a person type)
- Membership
- Member grants

# Additional Changes
minor grammatical edits in the docs

# Additional Context
- Closes https://github.com/zitadel/zitadel/issues/11291

---------

Co-authored-by: Marco A. <[email protected]>

apps/docs/content/concepts/structure/administrators.mdx

@@ -0,0 +1,13 @@
+---
+title: Administrators
+sidebar_label: Administrators
+---
+
+import AdministratorDescription from "./_administrator_description.mdx";
+
+<AdministratorDescription components={props.components}  name="AdministratorDescription" />
+
+Notes:
+
+- Read our [guide on Administrators](../../guides/manage/console/administrators) to learn more about the role concept and how to use Administrator roles in ZITADEL.
+- [API reference](/apis/resources/mgmt/management-service-list-org-member-roles) for Administrators on the organization level