Link to the Firefox delta graphs from the front page (instead of directly to the json file)

[whimboo]

With the changes from #25 we got two new graphs added:

• https://puppeteer.github.io/ispuppeteerwebdriverbidiready/?firefox-delta
• https://puppeteer.github.io/ispuppeteerwebdriverbidiready/?firefox-delta-all

Sadly both are not accessible from the front page. My suggestion would be to add a link to the delta page from the front page via the Firefox (WebDriver BiDi) has XXX failing or skipped tests. The link to the JSON file we could then show on that particular delta page only.

@OrKoN @juliandescottes