docs: fix broken links to project-management integration guides

[jtmchorse]

• I have read and reviewed the documentation changes to the best of my ability.
• If the change is significant, I have run the documentation site locally and confirmed it renders as expected.

Summary of changes

The three "Platform-Specific Setup Guides" links on the Project Management overview page currently 404.

They point at relative paths with a .md extension:

./jira-integration.md
./jira-dc-integration.md
./linear-integration.md

But the target files are .mdx (jira-integration.mdx, jira-dc-integration.mdx, linear-integration.mdx), and Mintlify serves those pages at extensionless routes — so none of the three links resolve.

These were also the only .md-suffixed internal page links in the repo; every other internal link is extensionless.

This PR points them at the correct absolute, extensionless routes, matching the convention in openhands/DOC_STYLE_GUIDE.md:

When linking other pages in the docs, always use absolute paths.

Only the three link targets changed — no copy or page content was touched.

🤖 Generated with Claude Code