added link to 30 days trial on platform.sh

[OskarStark]

No description provided.

[javiereguiluz]

@OskarStark I thank you for contributing to improve Symfony Docs ... but I'm 👎 about this change. The reason is that this link can be outdated soon (what if they create other promotions, or extend/reduce the trial period?)

I propose the following reword:

Before:

You need to subscribe to a `Platform.sh project`_ (or use a free `30 days trial`_). 
Choose the development plan and go through the checkout process. Once your project is +
ready, give it a name and choose: **Import an existing site**.

.. _`Platform.sh project`: https://accounts.platform.sh/platform/buy-now
.. _`30 days trial`: https://accounts.platform.sh/platform/trial/general/setup

After:

You need to `subscribe to a Platform.sh plan`_ and go through the checkout process.
Once your project is ready, give it a name and choose: **Import an existing site**.

.. _`subscribe to a Platform.sh plan`: https://accounts.platform.sh/platform/buy-now

[OskarStark]

@xabbuh are you fine with the proposal of @javiereguiluz ?
If yes, I will change it accordingly

[xabbuh]

@OskarStark fine for me

[OskarStark]

done

[xabbuh]

Thank you @OskarStark.