I have updated the `Documenting an Integration` documentation to be more
accurate on the usage of `tools/generate-integration-docs-screenshot` as
well as the requirement to fill out the `DOC_SCREENSHOT_CONFIG` section
of `zerver/lib/integrations.py` and a few other minor changes.
We're migrating to using the cleaner zulip.com domain, which involves
changing all of our links from ReadTheDocs and other places to point
to the cleaner URL.
