[web-php][PR #289] - Changing styles for navigation bar

You can view the Pull Request on github: https://github.com/php/web-php/pull/289 Opened By: SerhiiCho Pull Request Description: ### Intro Navigation bar is a little bit outdated for 2019, especially the mobile version. I think it's a good idea to make it look more **modern** for new people coming into the language. But at the same time don't make anything fancy and try to stick with the base structure. **Navigation bar** is a good place to start, but in future I'd like to continue with other parts as well. Just want to know your guys opinion about restyling [php.net](https://php.net) ### Search bar on mobile view The biggest change is that search bar is no longer visible on mobile view, instead there is a button with search icon on the left of the **navbar**. After first click it will show the search and **auto focus** on search input, but after second click it will try to **submit** the search form. ### Quick preview  _**Feel free to edit if you are a maintainer.**_