"API reference" link should be local to the build documentation

[twm]

On this page (built based on PR #11598) the "API reference" link points at https://twistedmatrix.com/documents/current/api/ but it should it be a relative link that points at the version that was built as part of the PR, https://twisted--11598.org.readthedocs.build/en/11598/api/index.html

[adiroiban]

Thanks.

Maybe also have this, since the docs are no longer hosted by us.
Maybe also add that the docs are hosted by Read The Docs

- An [API reference](https://twistedmatrix.com/documents/current/api/) is available on the twistedmatrix web site.
+ An [API reference](../) is available as part of the documentation site.

- An [API reference](https://twistedmatrix.com/documents/current/api/) is available on the twistedmatrix web site.
+ An [API reference](../) is available as part of the documentation site, hosted by Read The Docs.

[twm]

When viewing this page the user is already on Read the Docs so I'm not sure what that would add?