Refactor Secret Detection documentation to support multiple workflows

Problem to solve

Implementation Plan

As the Pre-receive SD moves to Beta and later to GA, we will want to do the following:

• Review the existing Secret Detection documentation and identify any improvements that can be made
• Retain the existing secret detection documentation page.
  • Reduce it to contain only the introduction of SD, including a brief description of both options: pre-receive and pipeline.
  • Cameron Swords did something similar for the DAST documentation a few months ago.
  • For DAST we explained there are two mutually exclusive options, but for SD we need to explain there are two complementary options.
• Create a new page - "Post-receive secret detection" in a subdirectory of the main page.
  • From the existing SD page, move everything from "After you enable SD..." to the new page.
  • Break the new page into "chunks" per the structure referenced in the OKR:
    • Each chunk is a child docs page of the main page.
    • Each chunk/file should be its own file, complete with metadata.
• Create a new page - "Pre-receive secret detection" in a subdirectory of the main page.
  • This new page's content should be as per the suggestion, "Create a new parent section titled Pre-receive SD".

Please see #431766 (comment 1672750333) for further information.