In this section of the docs, two tools for testing openapi
documentation are mentioned. But for the second one, we
forgot to mention that the tool also resides in the tools/
folder (like the first one which explicitly mentions it).
This commit fixes that.
Move docs/tutorials/documenting-api-endpoint.md to
docs/documentation/api.md.
This makes it easier to find when browsing the complete set of
materials on writing Zulip documentation.