[Site] Use ux_package template for ux-icons page

[karpilin]

Q	A
Bug fix?	no
New feature?	no
Issues	none
License	MIT

This changes /icons page to extend generic ux_packages/package template with the main goal to show composer require symfony/ux-icons in the page package-header section, just like on all other package pages.

before and after side-by-side:

[smnandre]

Hi @karpilin ! Thank you very much for your PR. We are changing some things (right now) about the header, so we won't merge your PR for now.

You will see later this week the small changes we're making on the package pages. Then it will be easier to discuss yours :)

This page is not that easy, as the vast majority of people visiting are here to look at icons, but i agree for people wanting to install the component it's a bit deceptive right now.

[karpilin]

Thanks for the feedback @smnandre

The only reason for the change was that it is not immediately obvious that a 'ux-icons' composer package has to be installed before icons can be used. I was just unifying it with other live components.

I'll keep an eye on the repo and will adjust my pr once the upcoming package page changes will be merged in.

[smnandre]

Hi @karpilin ! I'm coming back to you... we have had some very interesting discussion with the team, and the changes will need a few days to be done... i'm going back to you quickly !

[smnandre]

To let you know, i've had stuff to prepare for the symfony con. We'll meet there with the team and discuss things (website is one of them).

[karpilin]

Thanks for the update and enjoy the conference. Sadly won't be able to join you this year.

Let me know how you want me to go about this PR.
The change here is really minimal - it just shows composer require symfony/ux-icons in the page package-header section, making it more obvious how to start using the ux-icons library. The rest is exactly the same as it was.