Determine stage/group ownership of API and integration pages

To establish the stage group ownership of each doc page, we've been adding that info as front-matter at the top of each markdown file #216290 (closed) consistent with these guidelines: https://docs.gitlab.com/ee/development/documentation/#stage-and-group-metadata.

Here's a table of all API and integration pages.

1. API pages: Do we agree that they should each be owned by the stage/group that owns the respective features/resources for each? (Confirmed by @deuley)
   1. Who can label these relationships in the sheet? (Tech Writing team to proceed with ID-ing their respective group's resources, with @mikelewis to close gaps.)
   2. Proposed exceptions: Pages that apply to the API as a whole will be owned by Ecosystem. (Confirmed by @deuley)
2. Integration pages: Should all be owned by Ecosystem or are some integrations owned by other teams? (Update: per @deuley, all to be owned by Ecosystem)
3. Should any other topics/pages in our docs be owned by Ecosystem? (No such pages identified.)