refer to zulip chat as developer chat

[d-v-b]

Various places in the docs say something like "here is our zulip [link to zulip chat]". I think it's better to say "here is our developer chat [link to zulip chat]".
Two main reasons for this change:

• People might not know what zulip is. But they will know what developer chat is.
• We might hypothetically switch chat platforms. With these changes, we would only need to update the hyperlink.

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/user-guide/*.rst
• Changes documented as a new file in changes/
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)