Converts docs for `_cat/segments`, `_cat/plugins` and `_cat/repositories`
from `curl` to `// CONSOLE` so they are tested as part of the build and
are cleaner to use in Console. They should work fine with `curl` with
the `COPY AS CURL` link.
Also swaps the `source` type of the response from `js` to `txt` because
that is more correct. The syntax highlighter doesn't care. It looks at
the text to figure out the language. So it looks a little funny for `_cat`
responses regardless.
Relates to #18160
Nik Everett
Herbert Fischer
Clinton Gormley
Christoph Büscher