docs: Do not use relative URL to refer to integrations category.

[shubham-padia]

We are changing individual integration docs to live at integrations/{integration_name} instead of
integrations/doc/{integration_name}. Our relative link of ../version-control in this case will lead to /version-control on the root of the web app instead of intended
/integrations/version-control.

We are also changing categories to live at
/integrations/category/{category_name` instead of
/integrations/{category_name} on the web app. But the web app will be setting up redirects for existing categories to lead them to the new URL. This commit specifies the /integrations/{category_name} temporarily so that this PR can be merged before the changes in the web app and we don't have worry about syncing the releases between the web app and python-zulip-api.

Once the webapp code is merged, we should change all the relevant links in this repo to use the new URL structure.

How did you test this PR?

Self-review checklist

• Self-reviewed the changes for clarity and maintainability
  (variable names, code reuse, readability, etc.).

Communicate decisions, questions, and potential concerns.

• Explains differences from previous plans (e.g., issue description).
• Highlights technical choices and bugs encountered.
• Calls out remaining decisions and concerns.
• Automated tests verify logic where appropriate.

Individual commits are ready for review (see commit discipline).

• Each commit is a coherent idea.
• Commit message(s) explain reasoning and motivation for changes.

Completed manual review and testing of the following:

• Visual appearance of the changes.
• Responsiveness and internationalization.
• Strings and tooltips.
• End-to-end functionality of buttons, interactions and flows.
• Corner cases, error conditions, and easily imagined bugs.

[timabbott]

Merged, thanks @shubham-padia!