Simplify testbed/deployment.mdx "Configure your cloud access profile"

[ideaship]

The "Configure your cloud access profile" looks hard to follow for new users.

Potential pitfalls include:

• clouds-public.yaml mentioned without context: The text mentions clouds-public.yaml as if the user needs to do something with it, but it's actually pre-populated data for supported providers. Unclear what action (if any) the user should take.
• No guidance on application credential settings: When creating an application credential, the user may be given options (roles, unrestricted). The docs give no guidance on how to determine the appropriate options.
• No fast path for cloud access configuration: The section tries to cover different cases (app credentials vs username/password, secure.yaml, profiles, cloud naming constraints) without offering a simple straight path. A user on a standard OpenStack cloud gets bogged down in edge cases.
• Unclear what credentials a provider actually supplies: The docs say "The user-specific settings of the clouds.yaml file are provided by the cloud provider" but a standard OpenStack dashboard only shows User Name, User ID, Project Name, Project ID, and Authentication URL under "View Credentials" — none of which look like secrets. The relationship between what the dashboard shows and what goes into clouds.yaml is not explained.

This whole section should be rewritten (or split into multiple paths) to avoid overwhelming users.