Update debug Flutter application page

[polina-c]

This PR makes initial refresh of the summary at the top and deleted outdated content: https://github.com/flutter/website/pull/9830

The rest of the content should be removed (it is outdated and wrong) and replaced with explanation how to debug Flutter apps with VSCode.

New content may contain pieces of the deleted code from https://github.com/flutter/website/pull/9830, for example:

• analyser
• debug assertions
• common problems
• measuring app startup time

[christopherfujino]

@polina-c is there a timeline for this? There is some important debugging information that was deleted in https://github.com/flutter/website/pull/9830

[polina-c]

Thanks, @christopherfujino !

CC @kenzieschmoll

It seems this bug should be split between teams (analyzer, VS Code, DevTools), so that it gets proper attention and planning.