… doc (#23296)

Two link reference bugs caused by duplicate reference numbers:
- [16] was defined twice: once for the PHP library install URL and once
  for a video URL. The video placeholder line in the Overview was a
  leftover that served no purpose as a text link, so it was removed
  along with its duplicate [16] definition.
- [17] was defined twice: once in the PHP tab for the PHP library config
  URL and once at the bottom for the OpenAI API docs URL. The "OpenAI
  API Documentation" link in the API Key tab was incorrectly resolving
  to the PHP library docs. Fixed by assigning the OpenAI API docs URL
  to a new reference [21].

Also demoted heading levels in the API Key tab from h1/h2/h3 to
h4/h4/h5 to match the existing heading hierarchy (Installation is h3).