[Task]: Fix empty and copied page descriptions, and require a non-empty description in the docs check #8976

What needs to happen?

Follow-up to #2606. These weren't caught there: that pass, and the check added in #8957, only looked for a missing :description: line, so empty lines, placeholders and descriptions copied from another page went through.

1. Four pages have the :description: line, but with nothing after it:

   :description:
   

   so their meta description is empty: metadata-types/mail-server-connection, metadata-types/rest-connection, workflow/actions/repeat-end and workflow/actions/pipeline. "Check page descriptions" accepts them, because it only checks that the line is there.

2. Three pages carry another page's description: pipeline-data-probe has the one of PGP encrypt stream, the MQL action the one of the SQL action, and the Execution Information perspective the one of Execution Data Profile.

3. salesforce-connection, salesforce-fields and salesforce-settings are fragments included by the Salesforce transform pages, with a placeholder description "(not used directly )". Like the hop-conf fragments, they belong in pages/snippets/.

4. Make "Check page descriptions" also fail on a :description: line with no text after it.

Issue Priority

Priority: 3

Issue Component

Component: Documentation