urls: Simplify process to add URL redirects for documentation pages.

Creates `zerver.lib.url_redirects.py` to record old and new URLs
for documentation pages that have been renamed/moved and need URL
redirects.

This file is then used by `zproject.urls.py` to redirect links and
by `zerver.test.test_urls.py` to test that all of the old URLs
return a success response with a common page header/text depending
on the type of redirect (help center, policy, or API).

Adds a section to contributor docs on writing documentation for
how to use this redirect system when renaming a help center or api
documentation page.

Fixes #21946. Fixes #17897.

zerver/lib/test_helpers.py

@@ -481,24 +481,7 @@ def write_instrumentation_reports(full_suite: bool, include_webhooks: bool) -> N
             "confirmation_key/",
             "node-coverage/(?P<path>.+)",
             "docs/(?P<path>.+)",
-            "help/add-custom-emoji",
-            "help/configure-who-can-add-custom-emoji",
-            "help/change-the-topic-of-a-message",
-            "help/configure-missed-message-emails",
-            "help/community-topic-edits",
-            "help/about-streams-and-topics",
-            "help/delete-a-stream",
-            "help/add-an-alert-word",
-            "help/change-notification-sound",
-            "help/configure-message-notification-emails",
-            "help/disable-new-login-emails",
-            "help/test-mobile-notifications",
-            "help/troubleshooting-desktop-notifications",
-            "help/web-public-streams",
             "for/working-groups-and-communities/",
-            "help/only-allow-admins-to-add-emoji",
-            "help/night-mode",
-            "api/delete-stream",
             "casper/(?P<path>.+)",
             "static/(?P<path>.+)",
             "flush_caches",