In #23380 we want to change all occurrences of `uri` with `url`.
This commit changes the names of two variables `external_uri_scheme`
and `main_site_uri`, who are constructed using `settings` constants.
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.
Satyam Bansal
David Rosa
AcKindle3
Anders Kaseorg
Lauryn Menard