docs: Update links for other repository branch renames.

GitHub redirects these, but we should use the canonical URLs.

Signed-off-by: Anders Kaseorg <anders@zulip.com>
(cherry picked from commit 1ce12191aa)

docs/contributing/gsoc-ideas.md

@@ -391,7 +391,7 @@ CSS](https://github.com/zulip/zulip/).
   being built into Zulip.  And then for built-in bots, one should be able to click a few
   buttons of configuration on the web to set them up and include them in
   your organization.  We've developed a number of example bots
-  in the (`zulip_bots`](https://github.com/zulip/python-zulip-api/tree/master/zulip_bots)
+  in the [`zulip_bots`](https://github.com/zulip/python-zulip-api/tree/main/zulip_bots)
   PyPI package.
   **Skills recommended**: Python and JavaScript/CSS, plus devops
   skills (Linux deployment, Docker, Puppet etc.) are all useful here.

docs/contributing/zulipbot-usage.md

@@ -90,5 +90,5 @@ assignee(s) if they are still working on the issue.
 If you wish to help develop and contribute to **@zulipbot**, check out the
 [zulip/zulipbot](https://github.com/zulip/zulipbot) repository on GitHub and read
 the project's [contributing
-guidelines](https://github.com/zulip/zulipbot/blob/master/.github/CONTRIBUTING.md#contributing) for
+guidelines](https://github.com/zulip/zulipbot/blob/main/.github/CONTRIBUTING.md#contributing) for
 more information.

docs/documentation/api.md

@@ -237,7 +237,7 @@ above.
    bindings don't have a dedicated method for a specific API call,
    you may either use `client.call_endpoint` or add a dedicated
    function to the [zulip PyPI
-   package](https://github.com/zulip/python-zulip-api/tree/master/zulip).
+   package](https://github.com/zulip/python-zulip-api/tree/main/zulip).
    Ultimately, the goal is for every endpoint to be documented the
    latter way, but it's useful to be able to write working
    documentation for an endpoint that isn't supported by
@@ -285,7 +285,7 @@ above.
    in `zerver/openapi/zulip.yaml`, which mentions the API feature level
    at which they were added.
 
-[javascript-examples]: https://github.com/zulip/zulip-js/tree/master/examples
+[javascript-examples]: https://github.com/zulip/zulip-js/tree/main/examples
 
 ## Why a custom system?
 

docs/subsystems/events-system.md

@@ -138,7 +138,7 @@ soon as it attempted to send them to the client; if that specific HTTP
 response didn't reach the client due to a network TCP failure, then
 those events could be lost).
 
-[api-bindings-code]: https://github.com/zulip/python-zulip-api/blob/master/zulip/zulip/__init__.py
+[api-bindings-code]: https://github.com/zulip/python-zulip-api/blob/main/zulip/zulip/__init__.py
 
 The queue servers are a very high-traffic system, processing at a
 minimum one request for every message delivered to every Zulip client.

docs/subsystems/widgets.md

@@ -303,7 +303,7 @@ the **zform** to the client rendering it.
 
 
 First,
-[here](https://github.com/zulip/python-zulip-api/blob/master/zulip_bots/zulip_bots/bots/trivia_quiz/trivia_quiz.py)
+[here](https://github.com/zulip/python-zulip-api/blob/main/zulip_bots/zulip_bots/bots/trivia_quiz/trivia_quiz.py)
 is the code that produces the JSON.
 
 ``` py