integration-docs: Update the screenshot path of Jira plugin

[Niloth-p]

It was previously using the path corresponding to the directory name on python-zulip-api (jira). It's now updated to use the name as used in the zulip/zulip repository.

This PR is intended to be merged only after #34782.
Follow-up: Once this PR is merged, we should remove the exclusion added to test_integration_doc_endpoints in zulip/zulip's test_docs.py.

A previous version of the PR also updated Mercurial's hg/001.png -> mercurial/001.png, but I've since removed it, because of the hassle of tests failing whenever the images and docs are not in sync. So, I just added image_dir to point to hg in integrations.py.

It is unavoidable for the Jira plugin though, because jira/001.png is being used by the Jira webhook integration. And there's a difference in the template format used, between the webhook and plugin integrations. We could consider using the same image for both the integ docs only if we test and upate them both to use the same message formats.

Screenshots

Jira-plugin - before

Jira-plugin - after

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.
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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.