Updates the shared integrations documentation on generating a URL for
an incoming webhook for the new modal in the web-app, with links to the
help center. Also, links to a new section in the incoming webhooks
overview in the API documentation about the URL specification.
Also, adds a link to the help center in the shared webhooks integration
instructions for creating a stream for an incoming webhook. And updates
the example webhook URL to use a stream ID instead of a stream name.
Moves files in `templates/zerver/help/include` that are used
specifically for integrations documentation to be in a new
directory: `templates/zerver/integrations/include`.
Adds a boolean parameter to `render_markdown_path` to be used
for integrations documentation pages.
Lauryn Menard