Rename SD auto-response file, add redirect

What does this MR do?

Move Secret Detection post_processing.md to automatic_response.md to match changes made in !117087 (merged) and gitlab-com/www-gitlab-com!119911 (merged).

Related issues

• Rename secret "post-processing" to "automatic r... (!117087 - merged)
• Update description and name of automatic Secret... (gitlab-com/www-gitlab-com!119911 - merged)

Moving docs to a new location?

Read the guidelines: https://docs.gitlab.com/ee/development/documentation/index.html#move-or-rename-a-page

• Make sure the old link is not removed and has its contents replaced with a link to the new location.
• Make sure internal links pointing to the document in question are not broken.
• Search and replace any links referring to old docs in GitLab Rails app, specifically under the app/views/ and ee/app/views (for GitLab EE) directories.
• Make sure to add redirect_from to the new document if there are any Disqus comments on the old document thread.
  • ❓ This seems out-of-date. Removing this step in !119836 (merged).
• Update the link in features.yml (if applicable).
  • MR: gitlab-com/www-gitlab-com!124188 (merged)
• Assign one of the technical writers for review.