Add note about 'phar extension' dependency

[snoek09]

Q	A
Doc fix?	yes
New docs?	no
Applies to	all
Fixed tickets	#5919

[xabbuh]

👍

[javiereguiluz]

I agree with adding this note ... but I'm not sure about the wording.

In my opinion, the problem is that you are telling the users: "hey, do this and check this before executing the command". Problem is that most of our users won't have this issue (because PHAR is usually enabled) and a lot of newcomers don't know where or how to check for this "strange" phar extension. So we should avoid telling them to check anything before executing the command.

If we reword the note as follows, most of these issue would be solved:

If the installer doesn't work for you or doesn't output anything, make sure
that the `PHP phar extension`_ is installed and enabled in your computer.

[wouterj]

Thank you @snoek09 for adding this little helpfull note!

Now we changed the content of the note, I've moved it below the code example executing the symfony installer, as I think that makes more sense. I've also copied the note to the Installation documentation, where we mention the installer as well. See 709fc61.

Also, what do we think about adding a little command one can execute to check if the extension is available? E.g. php -r "echo extension_loaded('phar');".