VSCode: add recommended extensions, fix link to admonition styling CSS for markdown previews

[ewels]

I added in the settings file to point to the CSS file for docs admonition styling in markdown previews, back in 2024. But I made a copy-paste error so it never worked. See nf-core/website#2579 for original feature.

Fixing the path here to be a URL, that does something and actually works now (tested locally).

Also added a new extensions.json file that gets VS Code to show a little pop-up recommending that the user installs the listed plugins. I've done it as a slightly conservative sub-list of https://github.com/nf-core/vscode-extensionpack

These recommended extensions include Markdown Extended, which is needed for the admonition styling to work.

Side note: comments are tolerated in JSON files for VSCode. Tested locally and fine.

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.17%. Comparing base (cb623e0) to head (91fd0e7).

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[mahesh-panchal]

I had to catch up on the slack thread https://nfcore.slack.com/archives/C043UU89KKQ/p1787644811361209, because I thought this wasn't wanted. I guess it doesn't make a whole lot of difference though. It's useful for the people that can use it, and no difference to the people that don't.

Aside from the missing ChangeLog entry 👍🏽