Provide general feature and workflow matrix for plugins

While fixing some links in pulp-oci-images, I've came across a reference to "Workflows and Use Cases", which was a set of pages on pulpcore legacy docs. On docs migration, while moving some of these pages, we didnt preserve the notion of a workflow in pulpcore docs.

If I was reading the pulp-oci-images and got to this point (the link is in a section called "Try out a workflow"), I would expect to go to a page with small descriptions of some workflows and links to concrete plugin-implementation for them. And possibly a matrix table, similar to the matrix request for dependencies (https://github.com/pulp/pulpcore/issues/5573), but maybe a little harder to automate.

Problem

No definitive reference for available features and workflows across plugins.

Initial Approach

What I see as possible first steps are:

• formalize better what "feature" and "workflow" mean
• have a compreehensive grouped list of features and workflows.
• edit: gather existing material/discussion on this topic

As a starting point, I tried to organize the titles mentioned in the "Workflow and Use Cases" page linked above.

# definition
workflow: high-level usecases which relies on a set of features
feature: objective capability provided by Pulp
    - builtin: doesn't requires per-plugin-support. 
    - extensions: requires per-plugin-support.

# initial triage
workflows:
    - upload-publish
    - promotion 
features:
    - built-in:
        - user:
            - complex-filtering
            - exposing-content
            - labels
            - repo-versioning
        - admin:
            - repairing-pulp
            - reclaim-disk-space
            - plugin-removal
            - scheduling-tasks
            - troubleshooting
    - extension (requires plugin support):
        - domains-multi-tenancy
        - sign-metadata
        - import-export
        - alternate-content-sources
        - on-demand-downloading

Further steps

Having a compreehensive list of that, we can:

• Create a simple Features and Workflows page manually
• Create some system to semi-automate this matrix:
  E.g: each plugin repository can contain a feature-set manifesto which is used by pulp-docs to generate the matrix.
  This manifesto can include things like tech-preview status, version info (included, modified and deprecated), etc.

Start by reviewing docs/workflows/index.rst and the existing material or discussion referenced in the issue, including issue #5573. Clarify the definitions and scope of features and workflows, then outline the supported plugin information. Done means a definitive feature and workflow reference, with a matrix approach documented if automation remains in scope.