Improve Extensions.html page

[marble]

From Slack: https://typo3.slack.com/archives/C028JEPJL/p1635961736212900

Chris Müller: 6:48 PM I want to point to an issue, which annoys me very much. When clicking on docs.typo3.org on "Developing & Using Extensions" the page always scrolls down to the search field for 3rd party extensions. I find this not optimal: The left menu is out of view and the main scope seems to be searching some extension manual. For me, most of the time, I am looking for system extension manuals. So I always have to scroll back to the top. So my proposals to adjust this UX failure: Remove the auto-scrolling feature and

• Move the search field to the top of the page to give a fast possibility to look out for extension manuals, or
• Use a TOC on the top of the page, so the contents of the page (and so the search possibility) is visible at first glance. What do you think?

[marble]

Should be done with v4.7.6. However, it hasn't been tested because we don't use that page any more.