Improve documentation for `spp_repository_pipeline_access` project setting

The following discussions from !165626 (merged) should be addressed:

Use the `content` type in a policy to reference CI/CD configuration stored in a security policy
project. This allows you to use the same CI/CD configuration among multiple policies, reducing the
overhead of maintaining these configurations. For example, if you have a custom secret detection
CI/CD configuration you want to enforce in policy A and policy B, you can reference the same
configuration in both policies but it's stored as a single YAML file.

Prerequisites:

- Users triggering pipelines run in those projects on which a policy containing the `content` type
  is enforced must have at minimum read-only access to the project containing the CI/CD
  configuration.

  In GitLab 17.4 and later, you can grant the required read-only access by enabling the project
  setting **Scan execution policies**. Enabling this option allows the user who triggered the
  pipeline access to **only** the read the CI/CD configuration enforced by the scan execution
  policy.

@mcavoj This suggestion does not comply with the docs standards, but I think it helps explain the feature and the required permissions a little more. As I'm now on PTO please ask Ryan Lehmann for assistance from here.

@rlehmann1 By our docs standards, this would be written as two topics - concept and task. Here we've combined them into one. It's not ideal but we can polish this in a follow-up MR.

@rdickenson started a discussion:

    sppRepositoryPipelineAccessLabel: s__(
      'ProjectSettings|Grant access to the CI/CD files named in security policies.',
    ),
    sppRepositoryPipelineAccessHelpText: s__(
      'ProjectSettings|Allow users and tokens read-only access to fetch security policy configurations in this project to enforce policies. %{linkStart}Learn more%{linkEnd}.',

suggestion: @mcavoj Like my docs suggestion, please consider this a draft.

@rlehmann1 You may prefer to leave this just as it is, with is OK with me. 👍